fix(pd): keep KvClient watches alive after reconnect failures

[contrueCT]

Purpose of the PR

• close [Bug] PD KvClient watch can permanently stop after reconnect failure #3152

KvClient previously retried a failed watch only once. If that reconnect also failed, the
watch stopped permanently, and unexpected stream completion did not trigger recovery.

The reconnect path also exposed a lifecycle issue: AbstractClient.resetStub() invoked the
virtual close() method, so transport initialization on a KvClient dispatched to
KvClient.close() and marked the whole client closed.

Main Changes

• Separate AbstractClient transport cleanup from the overridable client lifecycle close.
• Track each exact-key or prefix watch as an independent subscription with its current observer.
• Retry onError, Leader_Changed, and unexpected onCompleted with a fixed delay until the
  watch recovers or the client closes.
• Deduplicate reconnect scheduling for the same observer and ignore stale observer callbacks.
• Stop the reconnect executor and invalidate active observers when KvClient.close() is called.
• Add deterministic regression coverage for repeated failures followed by recovery, leader
  changes, completion, deduplication, stale observers, prefix semantics, and close behavior.

This PR does not add event replay, protocol revisions, PD Server changes, or the Server-side
reconciliation tracked by #3151. Events emitted while a watch is disconnected are still not
replayed.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:
  • mvn -q -o test -pl hugegraph-pd/hg-pd-test -am -Dtest=KvClientTest -DfailIfNoTests=false -Drat.skip=true -Djacoco.skip=true (12 tests, 0 failures/errors)
  • mvn -q -o package -pl hugegraph-pd -am -DskipTests -Dmaven.javadoc.skip=true -Drat.skip=true -Djacoco.skip=true
  • mvn -q -o -f hugegraph-pd/hg-pd-client/pom.xml apache-rat:check
  • mvn -q -o -f hugegraph-pd/hg-pd-test/pom.xml apache-rat:check

The full local PD suite was also attempted. The common suite passed 83/83 and the core suite
passed 89/91 with 2 skipped. The client suite could not complete without a local PD service at
127.0.0.1:8686 (4 failures and 5 errors reported connection refused), which prevented the rest
suite from executing in that Maven run.

Repository-wide RAT is not a valid signal in this checkout because pre-existing ignored
.upgrade-artifacts files produce 68 unrelated unapproved-license entries; RAT passes for both
changed modules.

Does this PR potentially affect the following parts?

• Dependencies (add/update license info & regenerate_known_dependencies.sh)
• Modify configurations
• The public API
• Other affects: PD KV watch reconnect lifecycle
• Nope

Documentation Status

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

[imbajin]

Blocking: yes. Summary: The reconnect implementation has critical identity and failure-propagation risks that can silently stop watches and invalidate existing locks; permanent callback errors also retry without termination. Evidence: static review of KvClient.java and AbstractClient.java at head 772b3a5, with exact server lock/watch handling cross-checked.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 138 lines in your changes missing coverage. Please review.
✅ Project coverage is 38.34%. Comparing base (c9a646d) to head (3c2f4df).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../java/org/apache/hugegraph/pd/client/KvClient.java	0.00%	132 Missing ⚠️
...org/apache/hugegraph/pd/client/AbstractClient.java	0.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3157      +/-   ##
============================================
- Coverage     39.23%   38.34%   -0.89%     
- Complexity      264      424     +160     
============================================
  Files           771      771              
  Lines         65938    66024      +86     
  Branches       8759     8778      +19     
============================================
- Hits          25872    25320     -552     
- Misses        37310    37970     +660     
+ Partials       2756     2734      -22     

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[imbajin]

Blocking: yes. Summary: The current head addresses the previously reported watch reconnect, lock identity, terminal-error retry, and stale-observer issues; focused validation passes, but coverage checks remain failed and independent review evidence is incomplete. Evidence: mvn -pl hugegraph-pd/hg-pd-test -am -Dtest=KvClientTest -DfailIfNoTests=false test (18/18 passed), git diff --check passed; codecov/project and codecov/patch failed.