fix(core): self-heal etcd meta watch on transport reconnect

[dpol1]

Purpose of the PR

• close [Improvement] Re-register HStore schema cache clear listener after a Meta reconnect #3036

CachedSchemaTransactionV2 registers a JVM-global watch on the meta store so that a
schema change on one node clears the schema cache on the other nodes. That watch goes
through EtcdMetaDriver.listen / listenPrefix, which handed jetcd the bare
Consumer<WatchResponse> overload. That overload drops onError and onCompleted, so
when jetcd ends a watch (for example after a transport reconnect tears down the gRPC
stream) the listener stopped receiving events with no log line and no exception. The node
kept serving stale schema and nothing reported it. PdMetaDriver does not have this
problem because its KvClient already re-subscribes on error.

Main Changes

• EtcdMetaDriver.listen / listenPrefix now register a Watch.Listener instead of the
  bare Consumer overload. On onError or onCompleted the driver re-subscribes after a
  1s backoff on a daemon thread, which is the same recovery PdMetaDriver gets through
  KvClient. A WARN is logged before each re-subscribe, so the failure is now visible.
• Removed CachedSchemaTransactionV2.resetMetaListenerForReconnect(). It was a manual
  stopgap added in fix(server): sync hstore schema cache clears #3011 with no callers, and it never shipped in a tagged release. Now
  that the driver recovers on its own, the JVM-global register-once flag is correct to stay
  set, so the manual reset has nothing left to do. The lifecycle comment on
  metaEventListenerRegistered is updated to describe the self-heal behavior.
• The MetaDriver interface is unchanged, so neither implementor needs edits beyond
  EtcdMetaDriver.

Known limitation: the re-subscribe opens a fresh watch without a stored revision, so any
cache-clear events emitted during the short reconnect window are not replayed. This is the
same behavior as PdMetaDriver / KvClient. The change removes the permanent silent
failure. It does not add gap replay.

Verifying these changes

• Need tests and can be verified as follows:
  • New EtcdMetaDriverTest (JUnit + Mockito) mocks the jetcd Client/Watch, captures
    the registered Watch.Listener, and asserts that both onError and onCompleted
    trigger a re-subscribe (a second watch(...) call), and that onNext still reaches
    the consumer.
  • Local run: EtcdMetaDriverTest 4/4, CachedSchemaTransactionTest 18/18, and
    MetaManagerSchemaCacheClearEventTest 6/6 after the stopgap removal.
  • Red-green: commenting out the re-subscribe makes the three recovery tests fail;
    restoring it makes them pass.

Does this PR potentially affect the following parts?

• Dependencies
• Modify configurations
• The public API
• Other affects (typed here)
• Nope

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[imbajin]

Blocking: no. Summary: No obvious issues found in the current head. Evidence: git diff --check passed; EtcdMetaDriverTest passed (4 tests); latest GitHub checks passed.