## What changed
Update Seatbelt tests to call `create_seatbelt_command_args` directly with explicit filesystem and network policies, and remove the unused legacy-policy wrapper.
Reuse `FileSystemSandboxPolicy::read_only()` in read-only policy conversion and test setup instead of constructing equivalent policies inline or through legacy conversion.
GitOrigin-RevId: 2e2b3342425a3477986f41135caa18003b697fe4