refactor(tests): reorganize tests into unittest/ and e2e/ directories

- Move all unit tests from tests/ to tests/unittest/
- Add tests/e2e/ directory for end-to-end tests
- Update conftest.py for new test structure
- Add new tests for auth dependencies, policies, route injection
- Add new tests for run callbacks, create store, execution artifacts
- Remove obsolete tests for deleted persistence layer

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/tests/unittest/test_model_config.py

@@ -0,0 +1,30 @@
+from deerflow.config.model_config import ModelConfig
+
+
+def _make_model(**overrides) -> ModelConfig:
+    return ModelConfig(
+        name="openai-responses",
+        display_name="OpenAI Responses",
+        description=None,
+        use="langchain_openai:ChatOpenAI",
+        model="gpt-5",
+        **overrides,
+    )
+
+
+def test_responses_api_fields_are_declared_in_model_schema():
+    assert "use_responses_api" in ModelConfig.model_fields
+    assert "output_version" in ModelConfig.model_fields
+
+
+def test_responses_api_fields_round_trip_in_model_dump():
+    config = _make_model(
+        api_key="$OPENAI_API_KEY",
+        use_responses_api=True,
+        output_version="responses/v1",
+    )
+
+    dumped = config.model_dump(exclude_none=True)
+
+    assert dumped["use_responses_api"] is True
+    assert dumped["output_version"] == "responses/v1"