[Testing] Add CONTEXT operation validation

[dhegberg]

Migration from aws/aws-durable-execution-sdk-python-testing#50

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Context

Implement validation to prevent duplicate CONTEXT operations

Technical Requirements

• Implement validation to prevent duplicate CONTEXT operations
• Add "Cannot start a CONTEXT operation that already exists" error
• Ensure execution state consistency during validation failures
• Add validation for multiple CONTEXT operations with same ID

Implementation Details

• File: src/aws_durable_execution_sdk_python_testing/checkpoint/processors/context.py:39

Acceptance Criteria

• Validation implemented to prevent duplicate CONTEXT operations
• "Cannot start a CONTEXT operation that already exists" error added
• Execution state consistency ensured during validation failures
• Validation added for multiple CONTEXT operations with same ID