fix: guard guardian_command_source_tool_name with cfg(unix) (#16498)

This currently contributing to `rust-ci-full.yml` being red on `main`
for windows lint builds due to the cargo/bazel coverage gap that I'm
working on. Hopefully this gets us back on track.

codex-rs/core/src/guardian/approval_request.rs

@@ -160,6 +160,7 @@ fn command_assessment_action(
     }
 }
 
+#[cfg(unix)]
 fn guardian_command_source_tool_name(source: GuardianCommandSource) -> &'static str {
     match source {
         GuardianCommandSource::Shell => "shell",