The MQTT target spawned its rumqttc event loop exactly once through a
`OnceCell`. When the loop exited on a fatal protocol error, the finished
`JoinHandle` stayed in the cell, so `init()` never respawned it. The target
then went permanently silent: `publish` calls kept enqueuing but nothing was
ever delivered, while the target still looked healthy.
Replace the one-shot spawn with a supervisor task. The `OnceCell` now guards
a single supervisor that runs one event-loop session at a time and, when a
session exits, rebuilds the client and event loop from the latest
`MqttOptions` after an exponential backoff (1s..30s). A session that connected
resets the backoff so a transient drop reconnects promptly; repeated immediate
failures back off to avoid a reconnect storm. Cancellation from `close()` is
handled by the supervisor dropping the in-flight session future, so no
per-session cancel channel is needed.
Rebuilding options per session also keeps TLS hot-reload working across
reconnects. The backoff policy is a pure function and the supervisor loop is
generic over the session runner, so both are unit tested without a broker:
one test asserts the session restarts repeatedly until cancelled, another that
a live session stops promptly on cancel.
Refs: https://github.com/rustfs/backlog/issues/972
