[communication] Message counter can panic in shutdown

The message counter that uses a Rust MPSC can panic in shutdown if its attempt to report messages fails (presumably because the other side has hung up). It isn't immediately obvious why the other end is shutting down if there are still messages to read, so perhaps there is more to diagnose here, but we've seen this in the wild in tests. Some more discipline about error cases and intended invariants would help here.

The issue names no file or test. Locate the message counter and its Rust MPSC reporting path, then inspect how shutdown handles a failed send and whether the receiving side can hang up while messages remain. Reproduce the panic in an existing test scenario if available; done means shutdown no longer panics on this error case and the intended invariant is covered by regression testing.