Keep a RetryAfter halt until every backoff has finished - #5339

Closes #5338.

AIORateLimiter documents that a RetryAfter halts all requests for retry_after + 0.1 seconds. _retry_after_event is shared by the whole limiter, but it was set() in a finally that runs for every request — including requests that never cleared it. So any request already past inner()'s await self._retry_after_event.wait() when the halt began would release it on completion.

Two reachable consequences:

• an unrelated in-flight request completing ends the halt early (0.30s into a 2s halt, in the issue's script);
• with two requests backing off at once, the shorter backoff expiring releases the longer one.

Both need a request to be past the wait when the halt starts, which is the normal case under load — so the halt was weakest exactly when it mattered, and a 429 storm kept being fed.

Fix

Count the requests currently backing off and set the event only when that count reaches zero. A plain "only the clearer may set it" flag would fix the first case but not the second.

Tests

Two regression tests, both of which fail on master and pass here:

• test_retry_after_not_released_by_in_flight_request
• test_retry_after_not_released_by_shorter_backoff

They use a small ScriptedRequest helper mapping chat_id -> (latency, retry_after). The latency matters: the existing test_delay_all_pending_on_retry starts its second request after the halt is already in place, so that request blocks at the wait() and the bug is invisible. Reproducing it needs a request that is already in flight, which in turn needs the 429 to arrive after some delay, as it does in practice.

tests/ext/test_ratelimiter.py: 18 passed, 1 skipped. ruff check, ruff format --check and mypy are clean on both files.

No behaviour change other than restoring what the docstring already promises, so I have not touched the docs.