fix(sdk): Align exit paths

[ghost]

• Introduce ExecutionError and InvocationError for non-retriable and
  retriable errors respectfully.
• align callback waiting behaviour according to spec:
  • terminated without resuklt will raise an error
  • terminated and successful will return appropriate value
  • anything else will suspend execution

Issue #, if available:
fixes #45

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[Copilot]

Pull Request Overview

This PR refactors the exception hierarchy to introduce two distinct error types: ExecutionError for non-retriable errors that return FAILED status, and InvocationError for retriable errors that trigger Lambda retries. This aligns callback waiting behavior with the specification: callbacks that terminate without a result raise an error, successful terminations return a value, and pending callbacks suspend execution.

Key changes:

• Replaced FatalError with ExecutionError and InvocationError base classes
• Introduced UnrecoverableError base class with TerminationReason enum
• Updated callback result handling to align with spec for terminated vs pending operations

Reviewed Changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/aws_durable_execution_sdk_python/exceptions.py	Introduces new exception hierarchy with ExecutionError, InvocationError, UnrecoverableError, and TerminationReason enum
src/aws_durable_execution_sdk_python/execution.py	Updates error handling to differentiate between retriable (InvocationError) and non-retriable (ExecutionError) failures
src/aws_durable_execution_sdk_python/context.py	Aligns callback result handling with spec: raises CallbackError for missing operations and suspends for pending callbacks
src/aws_durable_execution_sdk_python/operation/callback.py	Replaces FatalError with CallbackError for missing callback details
src/aws_durable_execution_sdk_python/operation/child.py	Updates exception handling to check for InvocationError instead of FatalError
src/aws_durable_execution_sdk_python/operation/step.py	Replaces FatalError with ExecutionError for non-retriable failures
src/aws_durable_execution_sdk_python/operation/invoke.py	Replaces FatalError with ExecutionError for unreachable code paths
src/aws_durable_execution_sdk_python/serdes.py	Updates serialization/deserialization errors to use ExecutionError
src/aws_durable_execution_sdk_python/state.py	Adds is_cancelled() method to check operation status
tests/	Updates test files to use new exception types and verify correct behavior

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[ghost]

Last commit deals with merge conflicts ✌️