[Testing] Confirm Validation Logic with backend

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

---

## 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

Start by comparing the validators under src/aws_durable_execution_sdk_python_testing/checkpoint/validators with the backend validation logic, focusing on payload size checks. Done means all backend validator changes are identified and missing or outdated local validations are created or amended as appropriate.

Written by the indexing model from the issue text.