Release HTTP/2 semaphore permit on NoAvailableStreamIDError

[bysiber]

In the HTTP/2 implementation, handle_async_request / handle_request acquires a _max_streams_semaphore permit before getting a stream ID. If get_next_available_stream_id() raises NoAvailableStreamIDError, the exception handler correctly sets _used_all_stream_ids and decrements the request count, but it doesn't release the semaphore permit that was just acquired:

await self._max_streams_semaphore.acquire()      # permit consumed

try:
    stream_id = self._h2_state.get_next_available_stream_id()
    self._events[stream_id] = []
except h2.exceptions.NoAvailableStreamIDError:
    self._used_all_stream_ids = True
    self._request_count -= 1
    raise ConnectionNotAvailable()                # permit not released

This permanently leaks one semaphore permit. While the practical impact is limited since _used_all_stream_ids = True prevents further requests on this connection, it's still incorrect resource cleanup.

This adds the missing release() call in both the async and sync implementations.