fix(cli): prevent 'Press ENTER to continue...' on exit

When AsyncOpenAI clients are garbage-collected after the event loop
closes, their AsyncHttpxClientWrapper.__del__ tries to schedule
aclose() on the dead loop, causing RuntimeError: Event loop is closed.
prompt_toolkit catches this as an unhandled exception and shows
'Press ENTER to continue...' which blocks CLI exit.

Fix: Add shutdown_cached_clients() to auxiliary_client.py that marks
all cached async clients' underlying httpx transport as CLOSED before
GC runs. This prevents __del__ from attempting the aclose() call.

- _force_close_async_httpx(): sets httpx AsyncClient._state to CLOSED
- shutdown_cached_clients(): iterates _client_cache, closes sync clients
  normally and marks async clients as closed
- Also fix stale client eviction in _get_cached_client to mark evicted
  async clients as closed (was just del-ing them, triggering __del__)
- Call shutdown_cached_clients() from _run_cleanup() in cli.py

cli.py

@@ -498,6 +498,14 @@ def _run_cleanup():
         shutdown_mcp_servers()
     except Exception:
         pass
+    # Close cached auxiliary LLM clients (sync + async) so that
+    # AsyncHttpxClientWrapper.__del__ doesn't fire on a closed event loop
+    # and trigger prompt_toolkit's "Press ENTER to continue..." handler.
+    try:
+        from agent.auxiliary_client import shutdown_cached_clients
+        shutdown_cached_clients()
+    except Exception:
+        pass
 
 
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