feat(image-gen): actionable setup message when no FAL backend is reachable (#26222)

When the in-tree FAL path has no API key (and no managed gateway), the
handler used to return a bare 'FAL_KEY environment variable not set'
error. Users had no idea where to get a key, that a managed Nous
gateway exists, or that plugin-registered providers are an option.

Now `image_generate_tool` returns a structured multi-line message:
  - signup link (https://fal.ai)
  - managed-gateway status (if Nous tools are enabled)
  - pointer to `hermes tools` / `hermes plugins list` for alternate
    backends, so users on a stale `image_gen.provider` know where to look

The schema is untouched — `check_fn` still gates the tool out of the
schema when no backend is reachable at startup, consistent with every
other conditional tool. This patch fixes the call-time failure modes:
managed-gateway 5xx, plugin provider disappearing mid-session, etc.

Inspired by #2546 / @Mibayy. The PR was ~5700 commits stale against
the new plugin-aware image_gen architecture, so this is a forward port
of the actionable-error idea rather than a cherry-pick.


Closes #2543

Co-authored-by: Mibayy <mibayy@users.noreply.github.com>

tests/tools/test_image_generation_env.py

@@ -37,3 +37,62 @@ def test_fal_key_empty_is_unset(monkeypatch):
     )
 
     assert image_generation_tool.check_fal_api_key() is False
+
+
+# ---------------------------------------------------------------------------
+# Actionable setup message when no FAL backend is reachable.
+# Regression for the silent-drop UX gap described in issue #2543.
+# ---------------------------------------------------------------------------
+
+
+def test_no_backend_message_mentions_fal_signup_and_plugins(monkeypatch):
+    from tools import image_generation_tool
+
+    monkeypatch.setattr(
+        image_generation_tool, "managed_nous_tools_enabled", lambda: False
+    )
+
+    msg = image_generation_tool._build_no_backend_setup_message()
+
+    assert "FAL_KEY" in msg
+    assert "https://fal.ai" in msg
+    # Plugin pointer so users on a stale image_gen.provider know where to look.
+    assert "hermes tools" in msg or "hermes plugins" in msg
+
+
+def test_no_backend_message_mentions_managed_gateway_when_enabled(monkeypatch):
+    from tools import image_generation_tool
+
+    monkeypatch.setattr(
+        image_generation_tool, "managed_nous_tools_enabled", lambda: True
+    )
+
+    msg = image_generation_tool._build_no_backend_setup_message()
+
+    assert "managed FAL gateway" in msg
+    assert "Nous account" in msg or "hermes setup" in msg
+
+
+def test_image_generate_tool_returns_actionable_error_when_no_backend(monkeypatch):
+    """End-to-end: handler must surface the actionable message, not a bare string."""
+    import json
+
+    from tools import image_generation_tool
+
+    monkeypatch.setattr(
+        image_generation_tool, "fal_key_is_configured", lambda: False
+    )
+    monkeypatch.setattr(
+        image_generation_tool, "_resolve_managed_fal_gateway", lambda: None
+    )
+    monkeypatch.setattr(
+        image_generation_tool, "managed_nous_tools_enabled", lambda: False
+    )
+
+    result = json.loads(
+        image_generation_tool.image_generate_tool(prompt="a cat")
+    )
+
+    assert result["success"] is False
+    assert "https://fal.ai" in result["error"]
+    assert "FAL_KEY" in result["error"]