Implement SendCallbackFailure

[yaythomas]

Context

Replace placeholder with actual callback failure logic

Technical Requirements

• Replace placeholder with actual callback failure logic
• Find and fail corresponding callback operation with error
• Update operation status to FAILED

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/executor.py:541

Acceptance Criteria

• Placeholder replaced with actual callback failure logic
• Corresponding callback operation found and failed with error
• Operation status updated to FAILED