fix: infer missing streaming tool call indexes

[pragnyanramtha]

Summary

• infer a missing streaming tool-call delta index when there is only one unambiguous target tool call
• cover the first tool-call delta omitting index, matching Gemini OpenAI-compatible streaming responses
• cover follow-up argument deltas omitting index while continuing the sole active tool call

Fixes #2806.

Context

Some OpenAI-compatible streaming providers omit delta.tool_calls[].index. The streaming chat state later indexes into accumulated tool calls with that value, which can raise TypeError: list indices must be integers or slices, not NoneType before user code sees the stream.

This patch only infers an index when the stream is unambiguous: a single incoming tool call with no prior tool call becomes index 0, and a single follow-up delta reuses the sole prior tool-call index. Multi-tool or ambiguous streams remain strict.

Validation

• uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot --with rich pytest tests/lib/chat/test_completions_streaming.py -k 'tool_call_delta_without_index' -p no:xdist -o addopts='' -> 2 passed, 15 deselected
• uv run --with pytest --with pytest-asyncio --with respx --with inline-snapshot --with rich pytest tests/lib/chat/test_completions_streaming.py -p no:xdist -o addopts='' -> 17 passed
• uv run --with ruff ruff check src/openai/lib/streaming/chat/_completions.py tests/lib/chat/test_completions_streaming.py -> passed
• uv run --with ruff ruff format src/openai/lib/streaming/chat/_completions.py tests/lib/chat/test_completions_streaming.py -> 2 files left unchanged
• git diff --check -> passed

[chatgpt-codex-connector]

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