fix: add debug for yarn binary path and version in _pick_yarn_cmd

npm_install.py

@@ -1989,7 +1989,11 @@ def _build_frontend(src_frontend: Path, dest_frontend: Path):
         """Find working yarn command (requires Yarn 4.x or higher)."""
         
         # Check yarn in PATH
-        if shutil.which("yarn"):
+        yarn_bin = shutil.which("yarn")
+        if yarn_bin:
+            result = subprocess.run([yarn_bin, "--version"], capture_output=True, text=True, timeout=5)
+            if DEBUG:
+                print(f"    debug pick: yarn_bin={yarn_bin}, version={result.stdout.strip()}, major={_get_yarn_major_version(['yarn'])}")
             major = _get_yarn_major_version(["yarn"])
             if major and major >= 4:
                 return ["yarn"]