finish wiring up cancel cause to Workflow cancellation.

[Quinn-With-Two-Ns]

finish wiring up cancel cause to Workflow cancellation. The client side is already done and the workflow already has access to this information so this PR just exposes it to workflow to workflow cancellation and tests it.

closes #2414

---

Note

Adds an optional cancellation reason to external workflow cancel APIs, plumbs it through SDK -> command attributes -> test server, derives from current CancellationScope when absent, and adds tests.

• API/Interfaces:
  • ExternalWorkflowStub: add cancel(@Nullable String reason); cancel() delegates.
  • WorkflowInternal: add cancelWorkflow(execution, @Nullable reason) overload.
  • WorkflowOutboundCallsInterceptor.CancelWorkflowInput: add optional reason.
  • ReplayWorkflowContext.requestCancelExternalWorkflowExecution(...): add @Nullable String reason param.
• Implementation:
  • Propagate reason into RequestCancelExternalWorkflowExecutionCommandAttributes and test server RequestCancelWorkflowExecutionRequest.
  • ExternalWorkflowStubImpl.cancel(...): derive reason from current CancellationScope if not provided.
  • Wire through SyncWorkflowContext.cancelWorkflow and ReplayWorkflowContextImpl.
• Tests:
  • Add WorkflowCancelReasonTest covering client-supplied, command-supplied, absent, and context-derived reasons.
• Misc:
  • Minor javadoc typo fix; update dummy/test contexts to new method signature.

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[maciejdudko]

One small comment but otherwise looks good, LGTM!

[maciejdudko]

This log should be either removed or made more informative.