Standardize Handling of None/Null CheckpointToken

[yaythomas]

Context

GitHub Reference: lambda_service.py#L833

Standardize how None/null CheckpointToken values are handled throughout the system. Align with TypeScript.

Technical Requirements

• Define consistent behavior for null checkpoint tokens
• Update all checkpoint token handling code
• Ensure proper validation and error messages
• Handle edge cases in token lifecycle

Implementation Details

• File: src/aws_durable_execution_sdk_python/lambda_service.py
• Line: Around line 833
• Validation: Add null checks and appropriate defaults

def handle_checkpoint_token(self, token: str | None) -> str:
    """Standardize checkpoint token handling."""
    if token is None:
        # Define standard behavior for null tokens
        logger.warning("Checkpoint token is None, using empty string")
        return ""

        # OR?
        # raise FatalError(f"Checkpoint token must be string, got {type(token)}")
    
    return token.strip()  # Normalize whitespace

Acceptance Criteria

• Align with TypeScript on behavior
• Consistent null token handling across codebase
• Clear validation and error messages
• Edge cases properly handled
• Behavior is documented
• Tests cover null token scenarios

Priority: Low
Estimated Effort: 0.5 day