🐛 Fix explicit response_model ignored on streaming generator endpoints - #16384

Pull Request

Discussion: #16382

Description

When specifying an explicit response_model (such as response_model=Item, response_model=Iterator[Item], or response_model=AsyncIterable[Item]) on generator streaming endpoints (JSONL and SSE), it was ignored during response serialization and generated an empty OpenAPI itemSchema: {}.

Root Cause

In _populate_api_route_state(), get_stream_item_type() was only called when response_model was an instance of DefaultPlaceholder. When an explicit response_model was provided:

1. route.stream_item_type was never populated, leaving stream_item_field as None.
2. route.response_field was erroneously populated instead.
3. At runtime, streaming endpoints serialize items via stream_item_field. Because it was None, serialization fell back to jsonable_encoder(), bypassing model validation and field filters (response_model_include, response_model_exclude, response_model_exclude_none).
4. OpenAPI generation emitted an empty itemSchema: {} because stream_item_field was absent.

Fix

In _populate_api_route_state():

• Check if the route is a streaming generator (is_sse_stream or is_json_stream) with an explicit response_model.
• Extract the item model using get_stream_item_type(response_model) or response_model.
• If it is not ServerSentEvent, assign route.stream_item_type = stream_item.
• Set response_model = None so stream_item_field is created for per-item serialization, validation, and OpenAPI generation.

Tests Added

Added tests/test_stream_response_model.py covering:

• Async JSONL stream with explicit model and field filtering (response_model_exclude, response_model_exclude_none)
• Async JSONL stream with response_model=AsyncIterable[Item]
• Sync generator JSONL stream with response_model=Iterator[Item]
• SSE stream with explicit model and field exclusion
• SSE stream yielding ServerSentEvent directly
• SSE stream with explicit response_model=ServerSentEvent
• Non-streaming endpoints unaffected
• Validation error raised on invalid yielded chunk in async stream
• Validation error raised on invalid yielded chunk in sync stream
• Router inclusion via app.include_router()
• OpenAPI schema generation (itemSchema for JSONL and contentSchema for SSE)

Checklist

• This PR links to a GitHub Discussion for the proposed code change.
• I added tests for the change.
• The new or updated tests fail on the main branch and pass on this PR.
• Coverage stays at 100%.
• The documentation explains the change if needed.