[Bug]: Virtual child replay can consume a non-virtual container checkpoint

## Expected Behavior

When replay changes a `run_in_child_context` operation from non-virtual to `ChildConfig(is_virtual=True)`, the SDK should reject the existing container checkpoint with `NonDeterministicExecutionError` before returning its cached result or emitting replay hooks.

Virtual child contexts never write their own container checkpoint. Therefore, any existing checkpoint at the virtual child operation ID proves that the prior execution used non-virtual checkpointing.

## Actual Behavior

`is_virtual` is not represented in `OperationIdentifier`. A successful `CONTEXT` checkpoint from a previous non-virtual child passes type, subtype, name, and parent validation. `ChildOperationExecutor.check_result_status()` then returns the cached result without executing the current virtual child body, so changes to the inner hierarchy are never validated.

This reproduces on both `main` and PR #698, so it is pre-existing and separate from #692, whose scope is type/subtype/name checkpoint identity.

## Suggested Fix

- Reject any existing child-container checkpoint when the current child is virtual.
- Perform the rejection before terminal checkpoint handling and replay/update plugin hooks.
- Add a unit regression for `ChildOperationExecutor` and a composed durable-execution test covering non-virtual to virtual replay.

## Context

Found while reviewing #698. Related review threads:

- #698 review comment 3909282767
- #698 review comment 3922629188

Beginne mit ChildOperationExecutor.check_result_status() und verfolge die Verarbeitung terminaler Checkpoints für virtuelle Child-Kontexte. Füge die angeforderte Unit-Regression und einen zusammengesetzten Durable-Execution-Test hinzu; abgeschlossen bedeutet, dass ein nicht virtueller Checkpoint mit NonDeterministicExecutionError abgelehnt wird, bevor gecachte Ergebnisse oder Replay-/Update-Hooks verwendet werden.

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