feat(xai): add xAI image generation provider (grok-imagine-image)

Add xAI as a plugin-based image generation backend using grok-imagine-image.
Follows the existing ImageGenProvider ABC pattern used by OpenAI and FAL.

Changes:
- plugins/image_gen/xai/__init__.py: xAI provider implementation
  - Uses xAI /images/generations endpoint
  - Supports text-to-image and image editing with reference images
  - Multiple aspect ratios (1:1, 16:9, 9:16, 4:3, 3:4, 3:2, 2:3)
  - Multiple resolutions (1K, 2K)
  - Base64 output saved to cache
  - Config via config.yaml image_gen.xai section
- plugins/image_gen/xai/plugin.yaml: plugin metadata
- tests/plugins/image_gen/test_xai_provider.py: 19 unit tests
  - Provider class (name, display_name, is_available, list_models, setup_schema)
  - Config (default model, resolution, custom model)
  - Generate (missing key, success b64/url, API error, timeout, empty response, reference images, auth header)
  - Registration

Requires XAI_API_KEY in ~/.hermes/.env.
To use: set image_gen.provider: xai in config.yaml.

tests/plugins/image_gen/test_xai_provider.py

@@ -0,0 +1,260 @@
+#!/usr/bin/env python3
+"""Tests for xAI image generation provider."""
+
+from __future__ import annotations
+
+import json
+import os
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+
+# ---------------------------------------------------------------------------
+# Fixtures
+# ---------------------------------------------------------------------------
+
+
+@pytest.fixture(autouse=True)
+def _fake_api_key(monkeypatch):
+    """Ensure XAI_API_KEY is set for all tests."""
+    monkeypatch.setenv("XAI_API_KEY", "test-key-12345")
+
+
+# ---------------------------------------------------------------------------
+# Provider class tests
+# ---------------------------------------------------------------------------
+
+
+class TestXAIImageGenProvider:
+    def test_name(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        assert provider.name == "xai"
+
+    def test_display_name(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        assert provider.display_name == "xAI (Grok)"
+
+    def test_is_available_with_key(self, monkeypatch):
+        monkeypatch.setenv("XAI_API_KEY", "sk-xxx")
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        assert provider.is_available() is True
+
+    def test_is_available_without_key(self, monkeypatch):
+        monkeypatch.delenv("XAI_API_KEY", raising=False)
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        assert provider.is_available() is False
+
+    def test_list_models(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        models = provider.list_models()
+        assert len(models) >= 1
+        assert models[0]["id"] == "grok-imagine-image"
+
+    def test_default_model(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        assert provider.default_model() == "grok-imagine-image"
+
+    def test_get_setup_schema(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        schema = provider.get_setup_schema()
+        assert schema["name"] == "xAI (Grok)"
+        assert schema["badge"] == "paid"
+        assert len(schema["env_vars"]) == 1
+        assert schema["env_vars"][0]["key"] == "XAI_API_KEY"
+
+
+# ---------------------------------------------------------------------------
+# Config tests
+# ---------------------------------------------------------------------------
+
+
+class TestConfig:
+    def test_default_model(self):
+        from plugins.image_gen.xai import _resolve_model
+
+        model_id, meta = _resolve_model()
+        assert model_id == "grok-imagine-image"
+
+    def test_default_resolution(self):
+        from plugins.image_gen.xai import _resolve_resolution
+
+        assert _resolve_resolution() == "1k"
+
+    def test_custom_model(self, monkeypatch):
+        monkeypatch.setenv("XAI_IMAGE_MODEL", "grok-imagine-image")
+        from plugins.image_gen.xai import _resolve_model
+
+        model_id, _ = _resolve_model()
+        assert model_id == "grok-imagine-image"
+
+
+# ---------------------------------------------------------------------------
+# Generate tests
+# ---------------------------------------------------------------------------
+
+
+class TestGenerate:
+    def test_missing_api_key(self, monkeypatch):
+        monkeypatch.delenv("XAI_API_KEY", raising=False)
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        provider = XAIImageGenProvider()
+        result = provider.generate(prompt="test")
+        assert result["success"] is False
+        assert "XAI_API_KEY" in result["error"]
+
+    def test_successful_generation(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.raise_for_status = MagicMock()
+        mock_resp.json.return_value = {
+            "data": [{"b64_json": "dGVzdC1pbWFnZS1kYXRh"}],  # base64 "test-image-data"
+        }
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp):
+            with patch("plugins.image_gen.xai.save_b64_image", return_value="/tmp/test.png"):
+                provider = XAIImageGenProvider()
+                result = provider.generate(prompt="A cat playing piano")
+
+        assert result["success"] is True
+        assert result["image"] == "/tmp/test.png"
+        assert result["provider"] == "xai"
+        assert result["model"] == "grok-imagine-image"
+
+    def test_successful_url_response(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.raise_for_status = MagicMock()
+        mock_resp.json.return_value = {
+            "data": [{"url": "https://xai.image/result.png"}],
+        }
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp):
+            provider = XAIImageGenProvider()
+            result = provider.generate(prompt="A cat playing piano")
+
+        assert result["success"] is True
+        assert result["image"] == "https://xai.image/result.png"
+
+    def test_api_error(self):
+        import requests as req_lib
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 401
+        mock_resp.text = "Unauthorized"
+        mock_resp.json.return_value = {"error": {"message": "Invalid API key"}}
+        mock_resp.raise_for_status.side_effect = req_lib.HTTPError(response=mock_resp)
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp):
+            provider = XAIImageGenProvider()
+            result = provider.generate(prompt="test")
+
+        assert result["success"] is False
+        assert result["error_type"] == "api_error"
+
+    def test_timeout(self):
+        import requests as req_lib
+
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        with patch("plugins.image_gen.xai.requests.post", side_effect=req_lib.Timeout()):
+            provider = XAIImageGenProvider()
+            result = provider.generate(prompt="test")
+
+        assert result["success"] is False
+        assert result["error_type"] == "timeout"
+
+    def test_empty_response(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.raise_for_status = MagicMock()
+        mock_resp.json.return_value = {"data": []}
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp):
+            provider = XAIImageGenProvider()
+            result = provider.generate(prompt="test")
+
+        assert result["success"] is False
+        assert result["error_type"] == "empty_response"
+
+    def test_with_reference_images(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.raise_for_status = MagicMock()
+        mock_resp.json.return_value = {
+            "data": [{"url": "https://xai.image/edited.png"}],
+        }
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp) as mock_post:
+            provider = XAIImageGenProvider()
+            result = provider.generate(
+                prompt="Edit this image",
+                reference_images=["https://example.com/ref1.png", "https://example.com/ref2.png"],
+            )
+
+        assert result["success"] is True
+        # Check that reference_images was passed in payload
+        call_args = mock_post.call_args
+        payload = call_args.kwargs.get("json") or call_args[1].get("json")
+        assert "reference_images" in payload
+        assert len(payload["reference_images"]) == 2
+
+    def test_auth_header(self):
+        from plugins.image_gen.xai import XAIImageGenProvider
+
+        mock_resp = MagicMock()
+        mock_resp.status_code = 200
+        mock_resp.raise_for_status = MagicMock()
+        mock_resp.json.return_value = {
+            "data": [{"url": "https://xai.image/test.png"}],
+        }
+
+        with patch("plugins.image_gen.xai.requests.post", return_value=mock_resp) as mock_post:
+            provider = XAIImageGenProvider()
+            provider.generate(prompt="test")
+
+        call_args = mock_post.call_args
+        headers = call_args.kwargs.get("headers") or call_args[1].get("headers")
+        assert "Bearer test-key-12345" in headers["Authorization"]
+        assert "Hermes-Agent" in headers["User-Agent"]
+
+
+# ---------------------------------------------------------------------------
+# Registration test
+# ---------------------------------------------------------------------------
+
+
+class TestRegistration:
+    def test_register(self):
+        from plugins.image_gen.xai import XAIImageGenProvider, register
+
+        mock_ctx = MagicMock()
+        register(mock_ctx)
+        mock_ctx.register_image_gen_provider.assert_called_once()
+        provider = mock_ctx.register_image_gen_provider.call_args[0][0]
+        assert isinstance(provider, XAIImageGenProvider)
+        assert provider.name == "xai"