fix(streaming): merge tool_call deltas by logical index, not physical position

[nileshpatil6]

Problem

accumulate_delta in src/openai/lib/streaming/_deltas.py looks up existing list entries using the \"index\" field as a physical list position (acc_value[index]). This breaks when a single streamed chunk contains multiple tool_calls entries that share the same logical index.

Servers that use speculative decoding (e.g. vLLM with Kimi K2.6) emit a first chunk like:

{
  \"delta\": {
    \"tool_calls\": [
      {\"index\": 0, \"id\": \"functions.list_files:0\", \"function\": {\"name\": \"list_files\"}, \"type\": \"function\"},
      {\"index\": 0, \"function\": {\"arguments\": \" {\\\"\"}}
    ]
  }
}

Because key not in acc is true on the first chunk, the list is stored as-is with two physical entries both carrying index: 0. Later chunks with index: 0 get merged into acc_value[0] (the first physical entry), leaving the second entry permanently stranded. The final arguments JSON is split across two objects and is invalid.

Fix

Instead of acc_value[index] (positional), build a logical index -> physical position map over acc_value before processing each delta chunk. Subsequent entries are matched by logical index through this map, so duplicates within the same chunk are merged into one entry and later deltas always land in the right place.

This matches the approach used by the Go SDK, which uses the index directly.

Fixes #3201

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve logical index ordering when adding new delta entries

Appending unseen logical indices to the end of acc_value breaks the invariant that list position matches each entry's index, so out-of-order first appearances (e.g., index 1 chunk arrives before index 0) produce misordered arrays. Downstream streaming code indexes these arrays by logical index (for example choice_snapshot.message.tool_calls[tool_call_chunk.index] in src/openai/lib/streaming/chat/_completions.py), which can then read the wrong tool call or raise IndexError for valid streams from providers that interleave indices. The previous insert(index, ...) behavior maintained positional alignment.

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