Read _reader under the lock in WindowsApiEmitter.queue_events() (#1170)

[uttam12331]

Summary

Closes #1170.

WindowsApiEmitter.on_thread_stop() reads and clears self._reader while holding self._lock:

def on_thread_stop(self) -> None:
    with self._lock:
        reader = self._reader
        self._reader = None
    if reader is not None:
        reader.stop()

but queue_events() read self._reader without holding the same lock, so the two accesses to the shared attribute were unsynchronized.

Fix

Copy self._reader into a local while holding self._lock, mirroring on_thread_stop(), then use the local reference outside the lock (the reader.get_events() loop stays outside the lock, as before). This is the approach suggested in the issue.

Tests

Added test_queue_events_reads_reader_under_lock, which spies on the emitter's lock and asserts that queue_events() acquires it while reading _reader. It fails on master (the lock is never taken in queue_events()) and passes with this change. ruff check/ruff format are clean; added a changelog entry.

(The test is in the Windows-only test_observers_winapi.py module and was run on Windows.)