fix(config): restore custom providers after v11→v12 migration

The v11→v12 migration converts custom_providers (list) into providers
(dict), then deletes the list. But all runtime resolvers read from
custom_providers — after migration, named custom endpoints silently stop
resolving and fallback chains fail with AuthError.

Add get_compatible_custom_providers() that reads from both config schemas
(legacy custom_providers list + v12+ providers dict), normalizes entries,
deduplicates, and returns a unified list. Update ALL consumers:

- hermes_cli/runtime_provider.py: _get_named_custom_provider() + key_env
- hermes_cli/auth_commands.py: credential pool provider names
- hermes_cli/main.py: model picker + _model_flow_named_custom()
- agent/auxiliary_client.py: key_env + custom_entry model fallback
- agent/credential_pool.py: _iter_custom_providers()
- cli.py + gateway/run.py: /model switch custom_providers passthrough
- run_agent.py + gateway/run.py: per-model context_length lookup

Also: use config.pop() instead of del for safer migration, fix stale
_config_version assertions in tests, add pool mock to codex test.

Co-authored-by: 墨綠BG <s5460703@gmail.com>
Closes #8776, salvaged from PR #8814

tests/hermes_cli/test_config.py

@@ -10,6 +10,7 @@ from hermes_cli.config import (
     DEFAULT_CONFIG,
     get_hermes_home,
     ensure_hermes_home,
+    get_compatible_custom_providers,
     load_config,
     load_env,
     migrate_config,
@@ -424,6 +425,146 @@ class TestAnthropicTokenMigration:
             assert load_env().get("ANTHROPIC_TOKEN") == "current-token"
 
 
+class TestCustomProviderCompatibility:
+    """Custom provider compatibility across legacy and v12+ config schemas."""
+
+    def test_v11_upgrade_moves_custom_providers_into_providers(self, tmp_path):
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text(
+            yaml.safe_dump(
+                {
+                    "_config_version": 11,
+                    "model": {
+                        "default": "openai/gpt-5.4",
+                        "provider": "openrouter",
+                    },
+                    "custom_providers": [
+                        {
+                            "name": "OpenAI Direct",
+                            "base_url": "https://api.openai.com/v1",
+                            "api_key": "test-key",
+                            "api_mode": "codex_responses",
+                            "model": "gpt-5-mini",
+                        }
+                    ],
+                    "fallback_providers": [
+                        {"provider": "openai-direct", "model": "gpt-5-mini"}
+                    ],
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            migrate_config(interactive=False, quiet=True)
+            raw = yaml.safe_load(config_path.read_text(encoding="utf-8"))
+
+        assert raw["_config_version"] == 17
+        assert raw["providers"]["openai-direct"] == {
+            "api": "https://api.openai.com/v1",
+            "api_key": "test-key",
+            "default_model": "gpt-5-mini",
+            "name": "OpenAI Direct",
+            "transport": "codex_responses",
+        }
+        # custom_providers removed by migration — runtime reads via compat layer
+        assert "custom_providers" not in raw
+
+    def test_providers_dict_resolves_at_runtime(self, tmp_path):
+        """After migration deleted custom_providers, get_compatible_custom_providers
+        still finds entries from the providers dict."""
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text(
+            yaml.safe_dump(
+                {
+                    "_config_version": 17,
+                    "providers": {
+                        "openai-direct": {
+                            "api": "https://api.openai.com/v1",
+                            "api_key": "test-key",
+                            "default_model": "gpt-5-mini",
+                            "name": "OpenAI Direct",
+                            "transport": "codex_responses",
+                        }
+                    },
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            compatible = get_compatible_custom_providers()
+
+        assert len(compatible) == 1
+        assert compatible[0]["name"] == "OpenAI Direct"
+        assert compatible[0]["base_url"] == "https://api.openai.com/v1"
+        assert compatible[0]["provider_key"] == "openai-direct"
+        assert compatible[0]["api_mode"] == "codex_responses"
+
+    def test_compatible_custom_providers_prefers_api_then_url_then_base_url(self, tmp_path):
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text(
+            yaml.safe_dump(
+                {
+                    "_config_version": 17,
+                    "providers": {
+                        "my-provider": {
+                            "name": "My Provider",
+                            "api": "https://api.example.com/v1",
+                            "url": "https://url.example.com/v1",
+                            "base_url": "https://base.example.com/v1",
+                        }
+                    },
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            compatible = get_compatible_custom_providers()
+
+        assert compatible == [
+            {
+                "name": "My Provider",
+                "base_url": "https://api.example.com/v1",
+                "provider_key": "my-provider",
+            }
+        ]
+
+    def test_dedup_across_legacy_and_providers(self, tmp_path):
+        """Same name+url in both schemas should not produce duplicates."""
+        config_path = tmp_path / "config.yaml"
+        config_path.write_text(
+            yaml.safe_dump(
+                {
+                    "_config_version": 17,
+                    "custom_providers": [
+                        {
+                            "name": "OpenAI Direct",
+                            "base_url": "https://api.openai.com/v1",
+                            "api_key": "legacy-key",
+                        }
+                    ],
+                    "providers": {
+                        "openai-direct": {
+                            "api": "https://api.openai.com/v1",
+                            "api_key": "new-key",
+                            "name": "OpenAI Direct",
+                        }
+                    },
+                }
+            ),
+            encoding="utf-8",
+        )
+
+        with patch.dict(os.environ, {"HERMES_HOME": str(tmp_path)}):
+            compatible = get_compatible_custom_providers()
+
+        assert len(compatible) == 1
+        # Legacy entry wins (read first)
+        assert compatible[0]["api_key"] == "legacy-key"
+
+
 class TestInterimAssistantMessageConfig:
     """Test the explicit gateway interim-message config gate."""
 
@@ -441,6 +582,6 @@ class TestInterimAssistantMessageConfig:
             migrate_config(interactive=False, quiet=True)
             raw = yaml.safe_load(config_path.read_text(encoding="utf-8"))
 
-        assert raw["_config_version"] == 16
+        assert raw["_config_version"] == 17
         assert raw["display"]["tool_progress"] == "off"
         assert raw["display"]["interim_assistant_messages"] is True