Fix: Correct streaming order for ReasoningItem and RawResponsesStreamEvent events

[ihower]

Issues

This PR fixes incorrect streaming order for ReasoningItem and RawResponsesStreamEvent:

1. RawResponsesStreamEvent should be emitted ASAP.

   This behavior was accidentally broken in Fix: Emit tool_called events immediately in streaming runs #1300, which changed the original correct ordering.

2. ReasoningItem should be emitted right after its response.output_item.done event, rather than being delayed until after response.completed. This issue was reported in ReasoningItem of RunStreamEvents are getting emitted out of order #1767 (comment)

Solution

1. Move raw event emission earlier

   Restored the correct ordering by moving RawResponsesStreamEvent to the top of the handler.

2. Emit ReasoningItem earlier

   Applied the exact same approach as in PR Fix: Emit tool_called events immediately in streaming runs #1300 for early emission of ToolCallItem, also applying it to ensure that ReasoningItem is emitted right after its ResponseOutputItemDoneEvent.

   Code intentionally not refactored for DRYness to keep the diff explicit and review-friendly. Refactoring can be done later after test code are merged.

3. Add comprehensive unit tests

   Extended FakeModel to fully simulate detailed streaming sequence and added tests verifying the exact event order, ensuring future changes won’t reintroduce regressions.

   For reference, here is a real example script that calls the OpenAI API, along with the corresponding 27 test events: https://gist.github.com/ihower/b6d9fe9b72b02c5131e210e2202b31b7

Related

• Fixes: ReasoningItem of RunStreamEvents are getting emitted out of order #1767 (comment)
• Fixes: Ordering of events in Runner.run_streamed is incorrect #583
• Related: Fix: Emit tool_called events immediately in streaming runs #1300

[waziers]

Thanks for fixing @seratch!!