fix(hardware): drain stdin in subprocess test to prevent broken pipe flake (#4161)

* fix(hardware): drain stdin in subprocess test to prevent broken pipe flake

The test script did not consume stdin, so SubprocessTool's stdin write
raced against the process exit, causing intermittent EPIPE failures.
Add `cat > /dev/null` to drain stdin before producing output.

* style: format subprocess test

src/hardware/subprocess.rs

@@ -407,7 +407,11 @@ mod tests {
         // Simpler: write a temp script.
         let dir = tempfile::tempdir().unwrap();
         let script_path = dir.path().join("tool.sh");
-        std::fs::write(&script_path, format!("#!/bin/sh\necho '{}'\n", result_json)).unwrap();
+        std::fs::write(
+            &script_path,
+            format!("#!/bin/sh\ncat > /dev/null\necho '{}'\n", result_json),
+        )
+        .unwrap();
         #[cfg(unix)]
         {
             use std::os::unix::fs::PermissionsExt;