Agent shell tool sandbox fails with "bwrap: setting up uid map: Permission denied"

Testing #334389

On Linux, enabling sandboxing prevents agent shell tool commands from starting.

### Repro steps

1. Enable shell tool sandboxing.
2. Run:
```bash
python environment_probe.py
```

### Actual result

The command exits with code 1 before Python starts:

```text
bwrap: setting up uid map: Permission denied
```

Running the same command with sandbox bypass succeeds.

### Expected result

The sandbox starts successfully and runs the command.