[INLONG-9484][Manager] Improve logic of sortstandalone sink auto-assign cluster

[vernedeng]

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• Fixes [Improve][Manager] Improve logic of sortstandalone sink auto-assigned cluster  #9484

Motivation

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Modifications

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Verifying this change

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• This change is a trivial rework/code cleanup without any test coverage.

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• This change added tests and can be verified as follows:

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  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

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