Implement thread-safe token sequence generation

[yaythomas]

Context

Add synchronization to Execution.get_new_checkpoint_token() method

Technical Requirements

• Add synchronization to Execution.get_new_checkpoint_token() method
• Use atomic operations for token sequence increment

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/execution.py:95

Acceptance Criteria

• Synchronization added to Execution.get_new_checkpoint_token() method
• Atomic operations used for token sequence increment