[JENKINS-49217] Export ChannelTermination#cause when it is an ExportedBean

[MinuteHanD]

Adds getCause() method to ChannelTermination that exports the cause field only when the exception has @ExportedBean annotation.
This allows the API to include detailed error information for exportable exceptions while maintaining backward compatibility.

See JENKINS-49217

Testing done

Added unit tests covering exportable exceptions, non-exportable exceptions, and null handling.

Proposed changelog entries

Export channel termination cause details when exception has @ExportedBean annotation

Proposed changelog category

/label bug

Proposed upgrade guidelines

N/A

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