[FLINK-40912][Connectors/Kafka] Close the exactly-once writer's AdminClient

[Savonitar]

What is the purpose of the change

Fix FLINK-40912: ExactlyOnceKafkaWriter creates an admin client for POOLING transaction cleanup but never closes it. Each writer restart can leak its network thread and associated resources.

Brief change log

• Close the lazily created admin client after the existing writer cleanup, preserving the cleanup order. The client remains scoped to the writer's lifetime.
• Add a package-private nullable getter that returns the existing client without creating one. Production callers retain lazy creation through getOrCreateAdminClient().
• Add separate integration tests for POOLING client cleanup and non-POOLING lazy initialization. POOLING's client accepts requests before writer closure and rejects them afterward with Kafka's shutdown exception and message. The non-POOLING test currently covers INCREMENTING and verifies that no client is created before or during closure.

Verifying this change

• Red-green verification: removing the admin-client cleanup makes the POOLING test fail because requests still succeed after writer closure. Fallback cleanup in the test prevents a leaked client when an assertion fails.
• All 30 tests in ExactlyOnceKafkaWriterTest, ExactlyOnceKafkaWriterITCase, and the two architecture test classes pass. Checkstyle and Spotless pass; frozen ArchUnit violation stores are unchanged.

Does this pull request potentially affect one of the following parts:

• Dependencies: no.
• Public API, Table options or PyFlink wrappers: no.
• Checkpointed state, serializers or exactly-once delivery: yes, the exactly-once writer's resource cleanup is affected. Checkpoint state, transaction selection and commit/abort behavior are unchanged.
• Per-record code paths: no.

Documentation

• New feature: no.
• Feature documentation: not applicable; the private field Javadoc is clarified.
• English and Chinese user documentation: not applicable.

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Was generative AI tooling used to co-author this PR?

• Yes

Generated-by: Codex (GPT-6)

[Efrat19]

Thanks for addressing this leak! Mostly LGTM

[Efrat19]

LGTM, thanks!