[Improvement] Support configurable unclean election for offline bucket recovery

### Search before asking

- [x] I searched in the [issues](https://github.com/apache/fluss/issues) and found nothing similar.

### Motivation

When a leader replica fails `NotifyLeaderAndIsr`, the Coordinator marks the replica offline and attempts a clean leader election.

Clean election only selects a live replica from the current ISR. If all ISR replicas repeatedly fail leader activation, the bucket can remain in `leader = -1` indefinitely, even when another live replica in the assignment is capable of becoming leader.

Currently, recovery mainly depends on subsequent cluster events or a Coordinator restart, which can retrigger leader election. This makes recovery unreliable for long-lived leaderless buckets.

The existing offline-leader retry mechanism also operates on offline replicas and may include ordinary follower `NotifyLeaderAndIsr` failures. Recovery should instead be scoped precisely to leaderless or offline buckets.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

- [x] I'm willing to submit a PR!