Implement some "wait for queue to empty" call

[johan-bjareholt]

Currently the example watcher waits for 1 second to make sure that the events are dispatched from the queue, would be nice if we could have a "wait for queue to empty" function which waits for the queue to empty if connected and instantly return if queue isn't running.

[TimeToBuildBob]

This is a reasonable API addition. The current QueuedActivityWatchClient sends heartbeats to a background thread that dispatches them, but there's no way for the caller to know when the queue is drained.

Use cases:

• Clean shutdown in watchers (ensure all events are persisted before exit)
• Testing (wait for events to be flushed before assertions)
• Batch operations (fill queue, wait for completion, then do something with the data)

Implementation approach:

class QueuedActivityWatchClient:
    def wait_for_flush(self, timeout: float = 10.0) -> bool:
        """Block until the dispatch queue is empty.
        Returns True if queue emptied, False if timeout reached."""
        if not self.connected:
            return True  # Nothing queued if not connected
        return self._dispatch_thread.queue.join(timeout=timeout)

The queue.Queue already supports join() — it blocks until all items have been processed (each task_done() call decrements the counter). The dispatch thread just needs to call task_done() after each successful send.

This is a small, self-contained change. The wait_for_flush name is clearer than "wait for queue to empty" since it communicates that the data is being sent (flushed) to the server, not just removed from the queue.

[rtsdque]

Hi! I'd like to work on this as part of my first open source contributions. I'll have a fix ready soon.