fix: merge streaming tool calls by logical index

[he-yufeng]

Summary

Fix the streaming delta accumulator so indexed lists are merged by their logical index field even when the first chunk already contains duplicate entries for the same index.

This keeps OpenAI-compatible streaming tool-call chunks like this from being stranded as two physical list entries:

• first entry: tool id/name at index: 0
• second entry: first argument fragment at index: 0
• later chunks: more argument fragments at index: 0

The chat-completions initial snapshot now runs the initial delta through the same accumulator path as later chunks, and the Assistants accumulator gets the same first-chunk handling.

Closes #3201.

To verify

$env:PYTHONPATH=(Resolve-Path .\src).Path; python -m pytest tests\lib\test_streaming_deltas.py tests\lib\chat\test_completions_streaming.py -q -o addopts=''
python -m py_compile src\openai\lib\streaming\_deltas.py src\openai\lib\streaming\_assistants.py src\openai\lib\streaming\chat\_completions.py tests\lib\chat\test_completions_streaming.py tests\lib\test_streaming_deltas.py
python -m ruff check src\openai\lib\streaming\_deltas.py src\openai\lib\streaming\_assistants.py src\openai\lib\streaming\chat\_completions.py tests\lib\chat\test_completions_streaming.py tests\lib\test_streaming_deltas.py
python -m ruff format --check src\openai\lib\streaming\_deltas.py src\openai\lib\streaming\_assistants.py src\openai\lib\streaming\chat\_completions.py tests\lib\chat\test_completions_streaming.py tests\lib\test_streaming_deltas.py
python -m pyright src\openai\lib\streaming\_deltas.py src\openai\lib\streaming\_assistants.py src\openai\lib\streaming\chat\_completions.py tests\lib\test_streaming_deltas.py
git diff --check

I also ran python -m pyright tests\lib\chat\test_completions_streaming.py; it still reports existing inline_snapshot unknown-type noise in that file, unrelated to this patch.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve positional fallback for unindexed assistant snapshots

For Assistants run steps, run_step_snapshots are seeded from thread.run.step.created with final RunStep tool-call objects, whose generated types do not include an index; only subsequent delta objects carry one. With this new lookup, an existing unindexed entry never matches delta_entry["index"], so the first delta for tool call 0 is appended as a duplicate instead of being merged into the existing slot. The event handler later reads step_snapshot.step_details.tool_calls[tool_call_delta.index], so code-interpreter/function deltas for created run steps can surface stale or empty snapshots rather than the accumulated arguments/input.

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