feat: Add buffered Chat Completions tool-call streaming

[incoffeemonster]

Summary

Some OpenAI-compatible Chat Completions providers can return valid function tool calls in the final assembled stream, but their streamed tool_calls chunks may not be reliable enough for the SDK to process incrementally. This can cause the Agents SDK to emit partial tool-call events too early or build invalid assistant/tool message ordering for later turns.

This PR adds an opt-in buffered streaming mode for those providers. When buffer_streamed_tool_calls=True is enabled, OpenAIChatCompletionsModel continues to stream normal text deltas immediately, but buffers function tool-call deltas until the provider stream completes. It then emits one complete synthetic tool-call chunk into the existing ChatCmplStreamHandler, so downstream Responses-format events and session history remain protocol-safe.

Default behavior is unchanged.

Key changes:

• Adds buffer_streamed_tool_calls to OpenAIChatCompletionsModel and OpenAIProvider.
• Adds openai_buffer_streamed_tool_calls passthrough to MultiProvider.
• Preserves usage chunks and provider-specific fields such as Gemini/LiteLLM thought signatures.
• Raises ModelBehaviorError when a stream finishes with finish_reason="tool_calls" but no tool-call deltas were observed.

Test plan

Automated checks run locally:

python3 -m py_compile src/agents/models/chatcmpl_stream_handler.py src/agents/models/openai_chatcompletions.py src/agents/models/openai_provider.py src/agents/models/multi_provider.py tests/models/test_openai_chatcompletions_stream.py

ruff check src/agents/models/chatcmpl_stream_handler.py src/agents/models/openai_chatcompletions.py src/agents/models/openai_provider.py src/agents/models/multi_provider.py tests/models/test_openai_chatcompletions_stream.py

ruff format --check src/agents/models/chatcmpl_stream_handler.py src/agents/models/openai_chatcompletions.py src/agents/models/openai_provider.py src/agents/models/multi_provider.py tests/models/test_openai_chatcompletions_stream.py

mypy src/agents/models/chatcmpl_stream_handler.py src/agents/models/openai_chatcompletions.py src/agents/models/openai_provider.py src/agents/models/multi_provider.py tests/models/test_openai_chatcompletions_stream.py

pyright src/agents/models/chatcmpl_stream_handler.py src/agents/models/openai_chatcompletions.py src/agents/models/openai_provider.py src/agents/models/multi_provider.py tests/models/test_openai_chatcompletions_stream.py

pytest tests/models/test_openai_chatcompletions_stream.py

Result: 27 passed.

Manual smoke test:

• Loaded the local fork via test.py.
• Confirmed agents imports from local src/agents.
• Confirmed OpenAIChatCompletionsModel(..., buffer_streamed_tool_calls=True).
• Verified normal text streaming.
• Verified a streamed tool-call run with DeepSeek completed without tool_calls/tool_call_id protocol errors.
• Verified two consecutive tool questions in the same SQLiteSession completed without stale assistant/tool message ordering errors.

Issue number

N/A

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

[chatgpt-codex-connector]

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[incoffeemonster]

@codex review

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Codex Review: Didn't find any major issues. Delightful!

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