TASK.md

Task contract: bounded async batch fetcher with retries

Change only starter/fetcher.py.

• Implement async_batch_fetch(items: list[dict], fetch_fn: Callable[[dict], Awaitable[dict]], max_concurrency: int = 3, max_retries: int = 3) -> list[dict].
• Concurrency bounding:
  • At most max_concurrency calls to fetch_fn may be executing concurrently at any moment.
• Retry & Backoff contract:
  • fetch_fn returns {"status_code": int, "data": ...} or raises an exception.
  • If status_code in {429, 500, 502, 503, 504} or an exception is raised, retry up to max_retries times.
  • Permanent client errors (status 400..499 except 429) must NOT be retried.
  • On retry, back off by 0.01 * (2 ** attempt) seconds.
• Result collection:
  • Return a list of result dictionaries preserving input item order.
  • Each result dict must contain:
    • "item": original input item dict.
    • "success": bool (True if final status_code == 200).
    • "status_code": int | None.
    • "data": response data or None.
    • "attempts": int (total attempts made, starting at 1).
    • "error": error message string or None.
• Individual item failures must never raise uncaught exceptions to abort other items.

Done means python examples/async-fetcher/verify.py starter exits successfully.