KAFKA-20914: Clear append and fetch purgatories when a leader resigns or becomes unattached

[anjy7]

Problem

When a KafkaRaftClient leader stops being the leader, its parked requests can no longer complete normally: pending appends can't gather the acknowledgments to commit at the old epoch, and held fetches should be failed so callers retry against the new leader. Both purgatories should therefore be completed exceptionally on any leader → non-leader transition. Only onBecomeFollower did this; the other two leader-exit paths did not.

So a leader that resigns, or drops to unattached (e.g. on seeing a higher epoch with no known leader), left pending appends. And, for unattached, pending fetches, sitting in purgatory until their request timeout expired.

Impact

The parked futures eventually expire via request.timeout.ms. But until then, callers wait the full timeout for a failure that is already certain, and each pending append future keeps its CompletedBatch (and backing ByteBuffer) pinned in memory for that duration.

Testing

• testResignWillCompleteAppendPurgatory and testTransitionToUnattachedWillCompleteAppendPurgatory
  (new): park an uncommitted append and assert the append purgatory goes from non-empty to empty
  across the transition (via a new test-only appendPurgatoryNumWaiting() accessor).
• testTransitionToUnattachedWillCompleteFetchPurgatory (new): asserts a held fetch is completed
  with NOT_LEADER_OR_FOLLOWER on the unattached path (mirrors the existing
  testResignWillCompleteFetchPurgatory).