fix(cli): fail sandbox exec when the relay closes without an exit status sandbox exec seeded its exit code to 0 and only overwrote it on an Exit event, so a stream that ended early, was cancelled, or was truncated reported a successful run. The gateway already treats the same condition as a relay failure (Status::unavailable); mirror that on the CLI side so exit 0 always means an observed exit status of 0. Fixes #2732 - #2957