Fix Gemini API content filter handling (#746)

## Summary
- avoid AttributeError when Gemini API returns `None` for chat message
- return empty output if message is filtered
- add regression test

## Testing
- `make format`
- `make lint`
- `make mypy`
- `make tests`

Towards #744

src/agents/models/openai_chatcompletions.py

@@ -71,12 +71,22 @@ class OpenAIChatCompletionsModel(Model):
                 stream=False,
             )
 
+            first_choice = response.choices[0]
+            message = first_choice.message
+
             if _debug.DONT_LOG_MODEL_DATA:
                 logger.debug("Received model response")
             else:
-                logger.debug(
-                    f"LLM resp:\n{json.dumps(response.choices[0].message.model_dump(), indent=2)}\n"
-                )
+                if message is not None:
+                    logger.debug(
+                        "LLM resp:\n%s\n",
+                        json.dumps(message.model_dump(), indent=2),
+                    )
+                else:
+                    logger.debug(
+                        "LLM resp had no message. finish_reason: %s",
+                        first_choice.finish_reason,
+                    )
 
             usage = (
                 Usage(
@@ -101,13 +111,15 @@ class OpenAIChatCompletionsModel(Model):
                 else Usage()
             )
             if tracing.include_data():
-                span_generation.span_data.output = [response.choices[0].message.model_dump()]
+                span_generation.span_data.output = (
+                    [message.model_dump()] if message is not None else []
+                )
             span_generation.span_data.usage = {
                 "input_tokens": usage.input_tokens,
                 "output_tokens": usage.output_tokens,
             }
 
-            items = Converter.message_to_output_items(response.choices[0].message)
+            items = Converter.message_to_output_items(message) if message is not None else []
 
             return ModelResponse(
                 output=items,

tests/test_openai_chatcompletions.py

@@ -191,6 +191,40 @@ async def test_get_response_with_tool_call(monkeypatch) -> None:
     assert fn_call_item.arguments == "{'x':1}"
 
 
+@pytest.mark.allow_call_model_methods
+@pytest.mark.asyncio
+async def test_get_response_with_no_message(monkeypatch) -> None:
+    """If the model returns no message, get_response should return an empty output."""
+    msg = ChatCompletionMessage(role="assistant", content="ignored")
+    choice = Choice(index=0, finish_reason="content_filter", message=msg)
+    choice.message = None  # type: ignore[assignment]
+    chat = ChatCompletion(
+        id="resp-id",
+        created=0,
+        model="fake",
+        object="chat.completion",
+        choices=[choice],
+        usage=None,
+    )
+
+    async def patched_fetch_response(self, *args, **kwargs):
+        return chat
+
+    monkeypatch.setattr(OpenAIChatCompletionsModel, "_fetch_response", patched_fetch_response)
+    model = OpenAIProvider(use_responses=False).get_model("gpt-4")
+    resp: ModelResponse = await model.get_response(
+        system_instructions=None,
+        input="",
+        model_settings=ModelSettings(),
+        tools=[],
+        output_schema=None,
+        handoffs=[],
+        tracing=ModelTracing.DISABLED,
+        previous_response_id=None,
+    )
+    assert resp.output == []
+
+
 @pytest.mark.asyncio
 async def test_fetch_response_non_stream(monkeypatch) -> None:
     """