[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.

src/aws_durable_execution_sdk_python_testing/checkpoint/processors/wait.py の 75 行目付近から始め、WAIT 操作とタイマーがどのように処理されているかを追跡します。既存のキャンセルフローを確認し、完了したキャンセルリクエストまたは無効なキャンセルリクエストがどのように表現されているかを特定します。列挙されたキャンセル、エラー処理、キャンセル後の実行に関する基準が満たされていれば完了です。

索引モデルが issue の本文から書いたものです。