Add token validation to get_execution_state

[yaythomas]

Context

Implement comprehensive checkpoint token validation

Technical Requirements

• Implement comprehensive checkpoint token validation
• Add token expiration checking
• Add proper error responses for invalid/expired tokens
• Handle cases where no token is provided

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/executor.py:362

Acceptance Criteria

• Comprehensive checkpoint token validation implemented
• Token expiration checking added
• Proper error responses added for invalid/expired tokens
• Cases where no token is provided handled

[yaythomas]

Note: might be an idea to rename used_tokens to generated_tokens