feat(codex): support explicit OpenAI Responses API config (#1235)

* feat: support explicit OpenAI Responses API config

Co-authored-by: Codex <noreply@openai.com>

* Update backend/packages/harness/deerflow/config/model_config.py

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Codex <noreply@openai.com>
Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/tests/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"