fix(memory): log CLI shutdown hook failures

Makes the CLI memory-provider shutdown path observable: log when CLI
cleanup calls memory shutdown (with session id + message count), warn
instead of swallowing CLI memory-shutdown exceptions, warn on
on_session_end failures during agent shutdown, and raise the
MemoryManager provider-hook failure log from debug to warning with a
traceback.

Salvaged from PR #49287 (authored by Gille / @helix4u).

agent/memory_manager.py

@@ -721,9 +721,10 @@ class MemoryManager:
             try:
                 provider.on_session_end(messages)
             except Exception as e:
-                logger.debug(
+                logger.warning(
                     "Memory provider '%s' on_session_end failed: %s",
                     provider.name, e,
+                    exc_info=True,
                 )
 
     def on_session_switch(