## Why
Concurrent callers can submit the same Windows sandbox setup payload while an
identical helper invocation is already running.
## What changed
- Share one in-flight helper invocation among setup and refresh requests with
identical serialized payloads.
- Return the shared success or failure to every waiter while preserving
structured setup error codes, then allow subsequent requests to run again.
## Testing
- Add a concurrency test that verifies identical requests execute the setup
operation once.
GitOrigin-RevId: 18ebbcb9b36de3eb5912b8bbbe153d39328b5e48