fix: use ResponseOutputRefusal for refusal content part added event

In the refusal handling branch, the `ResponseContentPartAddedEvent` was incorrectly
using `ResponseOutputText` as the `part` payload. This violates the OpenAI Agents
protocol, which requires refusal content to be represented by `ResponseOutputRefusal`
so that downstream consumers can access the `.refusal` field.

This change updates the `part` field to use `ResponseOutputRefusal(refusal="", type="refusal")`,
ensuring type consistency with both the accumulated state and the corresponding
`ResponseContentPartDoneEvent`.

Additionally, the `output_index` calculation is slightly improved for clarity by
explicitly using `(1 if state.reasoning_content_index_and_output else 0)` instead of
a boolean expression, though the primary fix is the payload type correction.

This is a safe, minimal, and high-impact fix that aligns the stream handler with
the official response schema.

Author: mshsheikh

src/agents/models/chatcmpl_stream_handler.py

@@ -309,12 +309,10 @@ async def handle_stream(
                     yield ResponseContentPartAddedEvent(
                         content_index=state.refusal_content_index_and_output[0],
                         item_id=FAKE_RESPONSES_ID,
-                        output_index=state.reasoning_content_index_and_output
-                        is not None,  # fixed 0 -> 0 or 1
-                        part=ResponseOutputText(
-                            text="",
-                            type="output_text",
-                            annotations=[],
+                        output_index=(1 if state.reasoning_content_index_and_output else 0),
+                        part=ResponseOutputRefusal(
+                            refusal="",
+                            type="refusal",
                         ),
                         type="response.content_part.added",
                         sequence_number=sequence_number.get_and_increment(),