🐛 fix(async): invoke _poll_delay during contended polling - #928

Problem

In BaseFileLock, contended lock acquisitions track an attempt counter and call self._poll_delay(poll_interval, attempt) to compute the retry delay (supporting exponential backoff, jitter, and subclass custom backoff hooks).

However, in BaseAsyncFileLock._async_poll_until_acquired (src/filelock/asyncio.py), the attempt counter was omitted and the delay was hardcoded as delay = poll_interval. As a result:

• Async locks (such as AsyncStrictSoftFileLock) and custom subclasses overriding _poll_delay never invoked _poll_delay during contended polling.
• The contention backoff behavior diverged between synchronous and asynchronous lock implementations.

Solution

1. Initialize attempt = 0 prior to the acquisition poll loop in BaseAsyncFileLock._async_poll_until_acquired.
2. Increment attempt += 1 on each retry attempt and pass it to self._poll_delay(poll_interval, attempt), mirroring BaseFileLock._poll_until_acquired.
3. Add regression test test_poll_contention_invokes_poll_delay in tests/test_async_filelock.py.
4. Include towncrier news fragment docs/changelog/+async-poll-delay.bugfix.rst.

Verification

• Confirmed failing test on base commit (AssertionError: BaseAsyncFileLock failed to call _poll_delay!).
• Ran full test_async_filelock.py suite (242 tests passing).
• Verified ruff check and ruff format --check pass cleanly.