fix(chat): handle out-of-order streamed choice indexes

[HostX0]

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Changes being requested

Fix Chat Completions stream accumulation when choice chunks arrive out of index order.

ChatCompletionStreamState currently uses choice.index as a Python list offset in the snapshot and event-state collections. That only works when every lower-numbered choice has already appeared. A chunk containing a single choice with index=1, for example, has a one-element choices list, so the initial snapshot path attempts to assign choices[1] and raises IndexError.

This change keeps the API-provided index as the choice identity instead of assuming it is the current list position:

• key per-choice event state by choice.index;
• resolve accumulated snapshots by their stored index field;
• keep newly discovered snapshots ordered by index for the final completion;
• transform the choices present in the initial chunk by their actual list positions rather than their logical indexes.

The regression test feeds the accumulator a valid 1 -> 0 -> 1 sequence and verifies that the later delta is appended to choice 1, while the final snapshot remains ordered as choices 0 and 1.

Additional context & links

The production change is confined to the handwritten src/openai/lib/streaming/chat/ implementation; no generated API resource or public model changes are required.