Audit cases of "tokio::mpsc::channel, size of 1"

For background, see: https://github.com/oxidecomputer/omicron/issues/9259

Any case where we create a tokio::mpsc::channel of size 1, we are subsequently vulnerable to deadlock with the following code:

loop {
  tokio::select! {
    // Suppose this is queued first, but is only returning `Poll::pending`
    _ = rx.send(...) => { return }
    // Suppose instead, this select! branch triggers...
    _ = yield_interval.tick() => {
      // ... Then this call to `send` will be stuck forever, queued behind the first rx.send,
      // which is no longer polled.
      let result = rx.send(...).await;
      if result = ... { return }
    }
  }
}

This issue will document cases where we're using mpscs, and risking this behavior.

Start by searching the repository for tokio::mpsc::channel calls configured with a size of 1. Review each matching case for the described select! and send behavior, then document the affected cases and any resulting deadlock risk. The issue does not name specific files or tests.