fix: avoid double-counting logprobs in streamed chat completions (#2444)

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request may not be merged

## 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.

Co-authored-by: Hayden <hayden@openai.com>