samples: workflow_streams: drop force_flush=True from charge_card

The activity's final publish was using force_flush=True, which sets the
flush_event so the background flusher fires immediately. Triggering a
flush right before __aexit__ runs the activity into the
WorkflowStreamClient's cancel-mid-flush path: __aexit__ cancels the
flusher task while it's awaiting the publish signal RPC, the cancel
propagates into the in-flight signal, and the activity hangs until the
StartToClose timeout fires. Empirically the workflow then retries the
activity indefinitely.

Without force_flush=True the buffered "card charged" event flushes via
the regular 200ms batch interval and the flusher is sleeping in
wait_for(...) when __aexit__ cancels it — a clean cancellation path.
The user-visible publish ordering is unchanged.

The underlying SDK bug should be fixed separately by switching __aexit__
from cancel() to a cooperative-stop flag so the in-flight signal
completes before the flusher exits.

Author: jssmith

workflow_streams/activities/payment_activity.py

@@ -26,6 +26,5 @@ async def charge_card(order_id: str) -> str:
         await asyncio.sleep(1.0)
         progress.publish(
             ProgressEvent(message="card charged"),
-            force_flush=True,
         )
     return f"charge-{order_id}"