feat: add checkpoint response handling

[yaythomas]

Description of changes:

Implement double-check pattern across all operation types to handle synchronous checkpoint responses, preventing invalid state transitions and unnecessary suspensions.

Bug fix: Callback operations now defer errors to Callback.result() instead of raising immediately in create_callback(), ensuring deterministic replay when code executes between callback creation and result retrieval.

Changes:

• Add OperationExecutor base class with CheckResult for status checking
• Implement double-check pattern: check status before and after checkpoint
• Use is_sync parameter to control checkpoint synchronization behavior
• Refactor all operations to use executor pattern:
  • StepOperationExecutor: sync for AT_MOST_ONCE, async for AT_LEAST_ONCE
  • InvokeOperationExecutor: sync checkpoint, always suspends
  • WaitOperationExecutor: sync checkpoint, suspends if not complete
  • CallbackOperationExecutor: sync checkpoint, defers errors to result()
  • WaitForConditionOperationExecutor: async checkpoint, no second check
  • ChildOperationExecutor: async checkpoint, handles large payloads
• Update all tests to expect double checkpoint checks with side_effect mocks

Affected modules:

• operation/base.py: New OperationExecutor and CheckResult classes
• operation/step.py: StepOperationExecutor implementation
• operation/invoke.py: InvokeOperationExecutor implementation
• operation/wait.py: WaitOperationExecutor implementation
• operation/callback.py: CallbackOperationExecutor with deferred errors
• operation/wait_for_condition.py: WaitForConditionOperationExecutor
• operation/child.py: ChildOperationExecutor with ReplayChildren support
• All operation tests: Updated mocks for double-check pattern

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[wangyb-A]

approved.
nit: The expression of using state.create_checkpoint in sync mode is different among different operations. Should we use create_checkpoint_sync to keep it consistent and easier to read?

e.g. we are using
self.state.create_checkpoint(operation_update=create_callback_operation) for callback. And using self.state.create_checkpoint(operation_update=start_operation, is_sync=True) for invoke.
They have same semantic though