Let a File call interrupt its request and still return the outcome

[SaladDay]

File protocol client change, split out of #349. A FUSE frontend that gets an interrupt for a waiting lock must learn whether the lock was acquired. Cancelling the call's context abandons the response, so the outcome is lost.

• With sandboxfs.WithInterrupt(ctx, interrupt), the client sends CancelRequest when interrupt closes and keeps waiting for the request's own response. The call returns the real result, or the service's failure with its effect.
• Ending the context still abandons the call, as before.
• The File access protocol doc states this in the client section.

There is no wire change. A new client test covers an interrupted request that still returns its result. go test -race ./internal/sandboxfs/... and vet pass.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}