[Testing] Implement WAIT operation cancellation (optional)

Migration from https://github.com/aws/aws-durable-execution-sdk-python-testing/issues/51

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## Context
Add WAIT operation cancellation support

## Technical Requirements
- Add WAIT operation cancellation support
- Implement timer cancellation when WAIT is cancelled
- Handle cancellation of already completed operations
- Add proper error handling for invalid cancellation requests
- Ensure execution continues properly after WAIT cancellation

## Implementation Details
- **File**: `src/aws_durable_execution_sdk_python_testing/checkpoint/processors/wait.py:75`

## Acceptance Criteria
- [ ] WAIT operation cancellation support added
- [ ] Timer cancellation implemented when WAIT is cancelled
- [ ] Cancellation of already completed operations handled
- [ ] Proper error handling added for invalid cancellation requests
- [ ] Execution continues properly after WAIT cancellation ensured

**Note**: This is marked as optional in the original document.