Preserve Pipe sink exception root causes

[Caideyipi]

Description

Preserve diagnostic information

• 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.

Test coverage

• Add unit coverage for null root-cause messages.
• Add unit coverage verifying that failure reporting preserves the original cause chain.

Testing

Not run locally because the machine does not have enough memory; CI will validate the change.

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This PR has:

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

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Key changed/added classes (or packages if there are too many classes) in this PR

• PipeAbstractSinkSubtask
• PipeSinkSubtask
• PipeRuntimeException and its critical sink subclasses
• PipeSinkSubtaskTest