[Testing] Confirm Validation Logic with backend

Migration from https://github.com/aws/aws-durable-execution-sdk-python-testing/issues/52

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## Context
Verify checkpoint validators accurately model the latest logic in the backend.

This includes payload size checks.

## Implementation Details
- **File**: `src/aws_durable_execution_sdk_python_testing/checkpoint/validators`

## Acceptance Criteria
- [ ] Compare existing validators to backend validators and identify new changes that are not in the local runner yet
- [ ] Create missing or amend existing validations as appropriate

首先，将 src/aws_durable_execution_sdk_python_testing/checkpoint/validators 下的验证器与后端验证逻辑进行比较，重点关注 payload 大小检查。完成的标准是识别出所有后端验证器变更，并适当创建或修改缺失或过时的本地验证。

由索引模型根据 Issue 内容生成。