[Bug]: Orphaned map/parallel branch can checkpoint after parent completion #640

Expected Behavior

When a map or parallel operation completes early because its completion
criteria are met, an in-flight branch marked as orphaned must not checkpoint
its terminal SUCCEED or FAIL result after the parent context completes.

Checkpoint admission should have a deterministic order:

• If the branch checkpoint is admitted first, it may be sent before the parent
  completion.
• If the parent completion marks the branch orphaned first, the branch
  checkpoint must be rejected and must not reach the checkpoint service.

Actual Behavior

ExecutionState.create_checkpoint() validates _parent_done while holding
_parent_done_lock, releases that lock, invokes the plugin hook, and only later
enqueues the update while holding _completion_lock.

This creates a check-to-enqueue race:

1. An in-flight branch calls create_checkpoint() with its terminal context
   SUCCEED update and passes the _parent_done check.
2. Before the branch update is enqueued, the map/parallel parent checkpoints
   SUCCEED and _mark_orphans() marks the branch operation ID as orphaned.
3. The branch resumes and enqueues its terminal SUCCEED update because orphan
   status is not checked again.
4. checkpoint_batches_forever() sends every queued update without
   revalidating orphan status.

A deterministic reproduction recorded two successful checkpoint-service calls:

service call 1: parent SUCCEED with BatchResult branch status STARTED
service call 2: branch SUCCEED with payload "branch-result"

Both synchronous checkpoint callers were released after the mock service
returned a new checkpoint token.

The customer-visible BatchResult remains replay-consistent in the current
implementation because normal payloads replay the parent's serialized result
and ReplayChildren summaries record startedIndexes. However, the durable
operation hierarchy can contain a terminal branch result checkpointed after
its parent context completed.

Relevant code:

• packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/state.py,
  ExecutionState.create_checkpoint() around the _parent_done_lock
  validation and later _checkpoint_queue.put()
• packages/aws-durable-execution-sdk-python/src/aws_durable_execution_sdk_python/state.py,
  checkpoint_batches_forever() where queued updates are sent without orphan
  revalidation

Steps to Reproduce

1. Create an ExecutionState with a branch operation registered as a child of
   a map/parallel parent.
2. Start a branch terminal CONTEXT/SUCCEED checkpoint on a worker thread.
3. Pause that thread after its initial orphan validation but before queue
   insertion, for example using a blocking synchronous plugin hook.
4. Checkpoint the parent CONTEXT/SUCCEED, causing _mark_orphans() to mark
   the branch operation ID.
5. Allow the parent update to be sent successfully.
6. Release the branch thread.
7. Observe that the branch terminal update is sent in a subsequent checkpoint
   API call and the synchronous branch caller completes successfully.

SDK Version

1.8.0, repository HEAD 7ac7acc6a7dae231f2abbb8e37f9780cc9b89af0

Python Version

3.14

Is this a regression?

No known regression.

Possible Fix

Make orphan validation and queue insertion atomic relative to parent
completion:

1. Inside the final _completion_lock block, acquire _parent_done_lock using
   a consistent global lock order.
2. Recheck whether operation_update.operation_id is in _parent_done
   immediately before queue insertion.
3. Optionally also reject when operation_update.parent_id is in
   _parent_done, preventing new durable descendants from being created under
   an already orphaned branch.
4. Insert the queued operation while still holding _parent_done_lock.

This produces deterministic ordering: either the child is queued first, or
parent completion wins and the child is rejected.

Add deterministic unit tests for:

• A terminal branch checkpoint paused between initial validation and enqueue.
• A new operation created under an already orphaned branch.
• The valid child-first ordering.
• Replay consistency for normal and ReplayChildren map/parallel results.

Additional Context

The existing core test suite passes (1504 passed, plus 5 subtests), but it
does not currently cover this interleaving.