fix(core): reject checkpoints at virtual child ids

[Thanatos9404]

Issue #, if available: Closes #704

Description of changes:

A virtual child context never writes a checkpoint of its own, so a checkpoint at its operation id can only come from a non-virtual child in an earlier invocation. is_virtual is not part of OperationIdentifier, so that checkpoint passed the identity checks and check_result_status() returned the cached result (or re-raised the cached error) without running the virtual body.

• ChildOperationExecutor.check_result_status(): for a virtual child, any checkpoint at its id raises NonDeterministicExecutionError ("... checkpoint contains a non-virtual child context but current child context is virtual", in the style of the FLAT message from fix(core): validate replay operation identity #698), after the identity check and before terminal handling. The check is the new ChildOperationExecutor.validate_virtual_checkpoint().
• DurableContext.run_in_child_context(): passes is_virtual through _operation_replay_aware() to _replay_aware(), which runs the same check right after validate_checkpoint(), before the replay or update plugin hook fires.

Outside replay, a virtual child or FLAT branch that finds a checkpoint at its id now fails with this error instead of using it. While replaying, FLAT branches still hit _validate_branch_checkpoint_nesting() first.

Testing:

• operation/child_test.py: a virtual child over a SUCCEEDED (with and without ReplayChildren), FAILED or STARTED checkpoint raises the error; the user function never runs and nothing is checkpointed.
• context_test.py: with a real PluginExecutor, a STARTED and an updated SUCCEEDED checkpoint fail before any replay or update hook.
• execution_test.py: a composed durable_execution test replays a non-virtual SUCCEEDED, FAILED or STARTED child as virtual and fails with NonDeterministicExecutionError; the body never runs and nothing is checkpointed.

All 9 new cases fail on main. The context test also fails with the _replay_aware check removed.

Locally (Windows, Python 3.13, Hatch 1.16.5): hatch run dev-core:cov reaches 98.28% (gate 98%), with context.py and child.py at 100%; the only failures are two Windows-only timestamp tests that also fail on main. hatch run dev-core:typecheck, the types:check script (mypy, all checked packages) and hatch fmt --check are clean. No conformance tests were added or changed.

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[zhongkechen]

/ai review

[github-actions]

Codex AI review

No actionable findings in the PR changes. Residual test risk: regression tests were inspected but not executed, as required by the review constraints.

Reviewed commit 86c58a702c0f5b9725abb91e565fd9ddf99f238c. Workflow run