Consolidate error handling

[yaythomas]

overview

Currently exceptions from a step raise as a CallableRuntimeError.

https://github.com/aws/aws-durable-execution-sdk-python/blob/3aea45710cc8da739f6d6ddb3b4120002b35f81d/src/aws_durable_execution_sdk_python/state.py#L188C1-L201C53

A recent refactor in Ts introduced some different/new error types.

ChildContextError Consistency:

• Always wrap errors in ChildContextError in executeChildContext
• Preserve original error message and pass error as cause
• Handle both Error objects and non-Error values consistently
• Add test for child context error with no Error field

StepError Consistency:

• Wrap generic errors in StepError at handler level
• Preserve DurableOperationError instances and specific error types
• Add test to verify Error objects are wrapped with correct message/cause

StepInterruptedError Handling:

• Pass StepInterruptedError to retry strategy for decision making
• Wrap StepInterruptedError in StepError when retry is exhausted
• Update tests to verify proper error wrapping and retry flow

per aws/aws-durable-execution-sdk-js#212

ac

1. Add ChiildContextError + implement in ChildContext handler
2. Add StepError + implement in Step handler (replacing CallabeRuntimeError)
3. StepInterruptedError raises as a StepError when retries exhausted