test: update test suite for PydanticAI integration

- Update conftest for lazy agent initialization
- Update chat router tests for Tatlock capabilities
- Update models router tests for tools capability
- Update responses advanced features tests
- Update main app tests
- Total: 131 tests, 81.78% coverage (up from 95 tests, 78.95%)

tests/models/test_router.py

@@ -24,7 +24,7 @@ def test_list_models(client: TestClient) -> None:
     # Check for expected model IDs
     model_ids = [m["id"] for m in data["data"]]
     assert "lorem-tester" in model_ids
-    assert "tatlock" in model_ids
+    assert "Tatlock" in model_ids
 
     # Verify model structure
     for model in data["data"]: