feat: report clarification dismiss ownership

[efecnc]

Summary

• add a backwards-compatible ClarificationHub API that atomically claims a pending dismissal
• preserve the existing cancellation method
• cover duplicate dismissal rejection

Test plan

• cargo test clarification::tests

[amazon-q-developer]

The implementation correctly adds atomic clarification dismissal ownership while maintaining backward compatibility. The refactoring of cancel_wait to delegate to cancel_wait_if_pending preserves existing behavior, and the new method properly uses DashMap's remove operation to atomically claim pending dismissals. Test coverage appropriately validates the single-claim behavior.

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