feature: support context validator

- We will need a context validator when user uses parent context in child handler.
- - ts ref: https://github.com/aws/aws-durable-execution-sdk-js/blob/development/packages/aws-durable-execution-sdk-js/src/utils/context-tracker/context-tracker.ts

Start by reading the referenced TypeScript implementation in packages/aws-durable-execution-sdk-js/src/utils/context-tracker/context-tracker.ts, then locate the corresponding Python context and handler entry points. Clarify how parent context is passed to a child handler and define tests showing that invalid context is rejected; done means the Python SDK supports context validation for this case.

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