fix: emit integer chat stream output indexes

[Aphroq]

Summary

Fixes Chat Completions streaming events so assistant message output_index values are emitted as integers instead of booleans when reasoning content precedes the message.

The stream handler now uses a shared helper to compute the assistant message output index and the reasoning-content regression test asserts that text/message events use 1 with exact int type, not True.

Test plan

• uv run pytest tests/models/test_reasoning_content.py -k 'stream_response_yields_events_for_reasoning_content'
• bash .agents/skills/code-change-verification/scripts/run.sh

Issue number

Closes #3109

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass

[Aphroq]

Closing as duplicate. #3108 now contains both Chat Completions streaming output_index fixes