[To dev/1.3] [Pipe] Preserve sink exception root causes (#18335)

[Caideyipi]

Description

Backport #18335 (8d5da83) to dev/1.3.

• Use Throwable.toString() for root-cause reporting so exceptions with a null message still include their exception type.
• Preserve the original throwable as the cause of PipeRuntimeSinkCriticalException instances.
• Add cause-aware constructors to the Pipe runtime exception hierarchy.
• Add unit coverage for null root-cause messages and preserved cause chains.

The dev/1.3 branch predates the newer master-side Pipe message layer and PipeSinkSubtask constructor shape, so the conflict resolution retains the existing 1.3 log templates and API shape while applying the diagnostic and cause-preservation changes.

Verification

• PASS: mvn -o -nsu spotless:apply -pl iotdb-core/node-commons,iotdb-core/datanode
• NOT COMPLETED: mvn -o -nsu -pl iotdb-core/node-commons,iotdb-core/datanode -Dtest=PipeSinkSubtaskTest -Dsurefire.failIfNoSpecifiedTests=false test
  • The local JVM failed native memory allocation in node-commons before the test executed because system virtual memory was exhausted. CI will validate the tests.

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This PR has:

• been self-reviewed.
• added or modified unit tests to cover the new code paths.