fix(test): use narrower mypy ignore code for method-assign

mypy now reports [method-assign] instead of [assignment] for
monkey-patched async methods. Update type: ignore comments to
use the narrower code so `make typecheck` passes cleanly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 4444J99

tests/mcp/test_connect_disconnect.py

@@ -161,8 +161,8 @@ async def tracked_exit_stack_aclose():
         call_order.append("exit_stack_closed")
         return await original_exit_stack_aclose()
 
-    server.session._write_stream.aclose = tracked_write_stream_close  # type: ignore[assignment]
-    server.exit_stack.aclose = tracked_exit_stack_aclose  # type: ignore[assignment]
+    server.session._write_stream.aclose = tracked_write_stream_close  # type: ignore[method-assign]
+    server.exit_stack.aclose = tracked_exit_stack_aclose  # type: ignore[method-assign]
 
     await server.cleanup()