fix: avoid double-counting logprobs in streamed chat completions

[dfedoryshchev]

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Changes being requested

#accumulateChatCompletion seeds a new choice with the chunk's logprobs by reference, so on that same iteration choice.logprobs === logprobs and choice.logprobs.content.push(...content) pushes the array into itself. One streamed token arrives as two entries in finalChatCompletion().choices[0].logprobs.content and in the logprobs.content.done payload.

The initialise branch then does choice.logprobs = { ...logprobs }, one level deep, so the snapshot keeps the caller's array and the next chunk's push appends into the chunk the caller passed in.

Current tests miss both: the fixture's first logprobs-bearing chunk is {"content":[],"refusal":null}, so push(...[]) is a no-op, and nothing streams two chunks that each carry logprobs.

Fix: seed logprobs: null, and copy the arrays when initialising.

Additional context & links

• Three added tests cover the content path, the refusal path and cross-chunk accumulation; all three fail on main.
• npx vitest run --config vitest.config.mts tests/lib/ChatCompletionStream.test.ts -> 38 passed. oxlint and oxfmt --check pass.