fix: merge duplicate streaming tool call deltas

[fengjikui]

Fixes #3201.

Summary

Streaming tool call deltas can contain multiple entries for the same index in a single chunk. The previous accumulator stored the first list delta as-is when the field was missing from the accumulator, so duplicate index entries from that initial chunk stayed split apart. A later delta for the same index then merged into the first physical list item, leaving earlier argument fragments stranded in a second item.

This changes list accumulation to use the same index-aware merge logic when a list field is first created, and shares that behavior between the chat and assistants streaming accumulators. It also adds a regression test covering duplicate tool_calls[0] entries in the initial accumulated list.

Validation

• Manual repro from Streaming tool_call deltas with duplicate indexes in first chunk are accumulated incorrectly #3201 now passes
• env -u ALL_PROXY -u HTTPS_PROXY -u HTTP_PROXY -u all_proxy -u https_proxy -u http_proxy .venv/bin/python -m pytest tests/lib/test_streaming_deltas.py tests/lib/chat/test_completions_streaming.py -q
• .venv/bin/python -m ruff format --check src/openai/lib/streaming/_deltas.py src/openai/lib/streaming/_assistants.py tests/lib/test_streaming_deltas.py
• .venv/bin/python -m ruff check src/openai/lib/streaming/_deltas.py src/openai/lib/streaming/_assistants.py tests/lib/test_streaming_deltas.py
• .venv/bin/python -m mypy src/openai/lib/streaming/_deltas.py src/openai/lib/streaming/_assistants.py tests/lib/test_streaming_deltas.py
• git diff --check

[fengjikui]

Closing this as a duplicate of #3425. #3425 covers the same #3201 issue and includes the broader initial chat completion snapshot path plus regression coverage, so keeping a single PR should make review easier.