Extract initial fetch error detection into reusable utility

[Copilot]

makeChatRequest2 provides no direct way to detect fetch-level errors (e.g. HTTP 404) before the stream begins. xtabProvider worked around this by manually wiring a DeferredPromise<void> + Promise.race() — boilerplate that's hard to follow and easy to get wrong.

Changes

• New createInitialFetchErrorDetector() in src/platform/chat/common/chatMLFetcher.ts
  Encapsulates the race-against-first-token pattern behind a clean, composable API:

  const { wrapFinishedCb, getInitialFetchError } = createInitialFetchErrorDetector();
  
  const fetchResultPromise = endpoint.makeChatRequest2({
    ...,
    finishedCb: wrapFinishedCb(myFinishedCb), // transparently signals first-token receipt
  }, token);
  
  const initialError = await getInitialFetchError(fetchResultPromise);
  if (initialError) {
    // fetch failed before any tokens arrived (e.g. 404, rate limit)
  }

  • wrapFinishedCb(cb) — decorates any FinishedCallback to signal when the first token arrives, passing all arguments through unchanged
  • getInitialFetchError(fetchResultPromise) — races the first-token signal against the full fetch result; returns the ChatFetchError if the fetch settles before any tokens, undefined otherwise

• src/extension/xtab/node/xtabProvider.ts
  Replaced the manual DeferredPromise<void> + inline Promise.race + firstTokenReceived.complete() in .finally with a single createInitialFetchErrorDetector() call. The resulting variable names (initialFetchError vs fetchRes) are also more self-documenting.

• src/platform/chat/test/common/chatMLFetcher.spec.ts (new)
  6 unit tests covering: first-token wins the race, fetch error before any tokens, fetch success before tokens, original callback delegation, undefined callback, and multiple error response types.

Original prompt

makeChatRequest2's API doesn't allow to get if the initial fetch errors (to handle 404) before we can get the stream.

Currently, this's handled by racing between firs token received and fetch result promise. However, I would prefer if we could extract this to a utility function with a nicer API.

Employ excellent engineering approach.

The user has attached the following file paths as relevant context:

• .github/copilot-instructions.md

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