Add Python-side shutdown timing logs for diagnosis

Add print statements with timestamps at key shutdown points
(initiate_shutdown, poller drain, finalize_shutdown) to correlate
with TEMP_FIX poll cancellation timestamps and determine if the
race is slow ShutdownWorker RPC or in-flight polls.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82

temporalio/bridge/sdk-core

@@ -837,7 +837,9 @@ async def raise_on_shutdown():
         )
 
         # Initiate core worker shutdown
+        logger.error("SHUTDOWN-DEBUG: initiate_shutdown called")
         self._bridge_worker.initiate_shutdown()
+        logger.error("SHUTDOWN-DEBUG: initiate_shutdown returned")
 
         # If any worker task had an exception, replace that task with a queue drain
         for worker, task in tasks.items():
@@ -853,7 +855,9 @@ async def raise_on_shutdown():
             self._nexus_worker.notify_shutdown()
 
         # Wait for all tasks to complete (i.e. for poller loops to stop)
+        logger.error("SHUTDOWN-DEBUG: awaiting poller loops")
         await asyncio.wait(tasks.values())
+        logger.error("SHUTDOWN-DEBUG: poller loops stopped")
         # Sometimes both workers throw an exception and since we only take the
         # first, Python may complain with "Task exception was never retrieved"
         # if we don't get the others. Therefore we call cancel on each task
@@ -871,7 +875,9 @@ async def raise_on_shutdown():
 
         # Do final shutdown
         try:
+            logger.error("SHUTDOWN-DEBUG: finalize_shutdown called")
             await self._bridge_worker.finalize_shutdown()
+            logger.error("SHUTDOWN-DEBUG: finalize_shutdown returned")
         except:
             # Ignore errors here that can arise in some tests where the bridge
             # worker still has a reference