[fix][client-cpp] Fail producers immediately when topic is terminated

[dwang-qm]

Fixes #566

Motivation

When a topic is terminated, producer operations should fail immediately with ResultTopicTerminated. Before this change, the C++ client could treat ResultTopicTerminated as retryable during producer creation and reconnection flows, which caused retries until the operation timed out instead of surfacing the terminal condition immediately.

Modifications

• Treat ResultTopicTerminated as a non-retryable producer error.
• Return the terminal result during producer creation and disconnection/reconnection handling.
• Reject later sends from a terminated producer with ResultTopicTerminated.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• Added ProducerTest.testCreateProducerAfterTopicTermination
• Added ProducerTest.testSendAfterTopicTerminationReconnect
• These tests are included in the standard CI unit-tests job.

Documentation

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• doc-not-needed
  This change only adjusts producer error handling for terminated topics and does not require user-facing documentation changes.

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