[server] Refactor CoordinatorEventProcessor and split its tests

### Search before asking

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

### Motivation

`CoordinatorEventProcessor` has accumulated responsibilities for table and partition lifecycle, server membership, replica and ISR transitions, rebalancing, and snapshot commits. Its size makes changes difficult to
review and increases the coupling between unrelated event handlers.

`CoordinatorEventProcessorTest` has grown alongside it and mixes tests for these different responsibilities. Temporary FileLength suppressions have been added for both files and should be removed after this refactoring.

Proposed work:
- Extract cohesive event-handling responsibilities into focused components within fluss-server, keeping CoordinatorEventProcessor responsible fororchestration and dispatch.
- Preserve execution on the Coordinator event thread, event ordering, fencing checks, error propagation, and ZooKeeper state transitions.
- Split CoordinatorEventProcessorTest into focused test classes aligned with the extracted responsibilities, reusing existing test infrastructure.
- Keep public APIs, RPC protocols, and persisted metadata formats unchanged.

### Solution

_No response_

### Anything else?

_No response_

### Willingness to contribute

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

Start by reading CoordinatorEventProcessor and CoordinatorEventProcessorTest in fluss-server, mapping each event-handling responsibility and its existing tests. Refactor the responsibilities into focused components while retaining orchestration, ordering, fencing, error propagation, and state transitions. Split the test class accordingly, remove the temporary FileLength suppressions, and verify that public APIs, RPC protocols, and persisted metadata formats remain unchanged.

Written by the indexing model from the issue text.