fix(mcp-image-gen): fix Heretic/FLUX2 integration bugs

- Fix syntax error in server.py (dangling docstring lines)
- Correct model filename: flux-2-klein-4b.safetensors (without -fp8)
- Fix _WORKFLOW_REGISTRY key to match actual downloaded filename
- Update get_models() to always include registry models as fallback
- Fix test expectations to match corrected model names
- All 37 tests passing

mcp/mcp-image-gen/tests/test_server.py

@@ -31,7 +31,7 @@ COMFYUI_BASE = "http://test-comfyui:8188"
 # ---------------------------------------------------------------------------
 
 def test_build_flux_workflow_structure():
-    """Verify build_flux_workflow returns a dict with correct node types."""
+    """Verify build_flux_workflow returns a dict with correct node types for default model."""
     wf = build_flux_workflow(
         prompt="a red cat",
         neg_prompt="ugly",
@@ -52,6 +52,47 @@ def test_build_flux_workflow_structure():
     assert wf["33"]["class_type"] == "CLIPTextEncode"
 
 
+def test_build_flux_workflow_heretic_model():
+    """Verify FLUX.2 Klein 4B with Heretic Qwen3-4B encoder uses correct nodes."""
+    wf = build_flux_workflow(
+        prompt="a red cat",
+        neg_prompt="ugly",
+        width=1024,
+        height=1024,
+        steps=4,
+        seed=42,
+        model="flux-2-klein-4b.safetensors",
+    )
+    assert wf["6"]["class_type"] == "CLIPTextEncode"
+    assert wf["30"]["class_type"] == "CLIPLoader"  # Qwen3-4B uses single CLIPLoader
+    assert wf["32"]["inputs"]["unet_name"] == "flux-2-klein-4b.safetensors"
+    assert wf["31"]["inputs"]["vae_name"] == "flux2-vae.safetensors"
+    assert wf["13"]["inputs"]["scheduler"] == "beta"  # FLUX.2 Klein uses beta scheduler
+
+
+def test_workflow_registry_contains_both_models():
+    """Verify the registry contains both supported models."""
+    assert "flux1-schnell.safetensors" in server._WORKFLOW_REGISTRY
+    assert "flux-2-klein-4b.safetensors" in server._WORKFLOW_REGISTRY
+    assert len(server._WORKFLOW_REGISTRY) == 2
+
+
+def test_workflow_registry_fallback():
+    """Unknown model falls back to default (FLUX.1-schnell)."""
+    wf = build_flux_workflow(
+        prompt="test",
+        neg_prompt="",
+        width=512,
+        height=512,
+        steps=4,
+        seed=42,
+        model="unknown-model.safetensors",
+    )
+    # Should have used default workflow (DualCLIPLoader)
+    assert wf["30"]["class_type"] == "DualCLIPLoader"
+    assert wf["32"]["inputs"]["unet_name"] == "unknown-model.safetensors"
+
+
 def test_build_flux_workflow_params_injected():
     """Verify all parameters are injected into correct nodes."""
     wf = build_flux_workflow(
@@ -202,14 +243,16 @@ async def test_list_available_models():
 @respx.mock
 @pytest.mark.asyncio
 async def test_list_available_models_comfyui_offline():
-    """When ComfyUI is unreachable, list_available_models returns error message."""
+    """When ComfyUI is unreachable, list_available_models falls back to registry models."""
     respx.get(f"{COMFYUI_BASE}/object_info/CheckpointLoaderSimple").mock(
         side_effect=httpx.ConnectError("connection refused")
     )
 
     result = await list_available_models()
-    assert len(result) == 1
-    assert "not reachable" in result[0].lower()
+    # Should return registry models even when ComfyUI is offline
+    assert isinstance(result, list)
+    assert "flux1-schnell.safetensors" in result
+    assert "flux-2-klein-4b.safetensors" in result
 
 
 # ---------------------------------------------------------------------------