A flaky test revealed that after `clients::stop()` was called,
the `old_clients` collection was sometimes not empty,
indicating that some clients were not being stopped correctly.
This resulted in sanitizer errors when objects went out of scope at the end of the test.
This patch modifies `stop()` to ensure all clients, including those in `old_clients`,
are stopped, guaranteeing a clean shutdown.
58456455e3