Remove the legacy Seatbelt command wrapper (#59774)

## 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

codex-rs/protocol/src/permissions.rs

@@ -2051,14 +2051,7 @@ impl From<&SandboxPolicy> for FileSystemSandboxPolicy {
         match value {
             SandboxPolicy::DangerFullAccess => FileSystemSandboxPolicy::unrestricted(),
             SandboxPolicy::ExternalSandbox { .. } => FileSystemSandboxPolicy::external_sandbox(),
-            SandboxPolicy::ReadOnly { .. } => {
-                FileSystemSandboxPolicy::restricted(vec![FileSystemSandboxEntry::new(
-                    FileSystemPath::Special {
-                        value: FileSystemSpecialPath::Root,
-                    },
-                    FileSystemAccessMode::Read,
-                )])
-            }
+            SandboxPolicy::ReadOnly { .. } => FileSystemSandboxPolicy::read_only(),
             SandboxPolicy::WorkspaceWrite {
                 writable_roots,
                 exclude_tmpdir_env_var,