[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.

Start with src/aws_durable_execution_sdk_python_testing/checkpoint/processors/wait.py around line 75 and trace how WAIT operations and timers are handled. Review the existing cancellation flow and identify how completed or invalid cancellation requests are represented. Done means the listed cancellation, error-handling, and post-cancellation execution criteria are covered.

Written by the indexing model from the issue text.