Preserve installer stderr when releasing the install lock (#59227)

## Why

Redirecting stderr on `exec` while closing the lock descriptor suppressed subsequent installer warnings, including the warning that an existing npm-managed Codex installation remains installed.

## What changed

Remove the stderr redirection from `release_install_lock` when closing file descriptor 9. Add a regression test asserting that a noninteractive install succeeds and emits the existing npm installation warning.

GitOrigin-RevId: 5a35ccd9bf62afb99cc949892ec724a01e2d9eee

scripts/install/install.sh

@@ -743,7 +743,7 @@ release_install_lock() {
   if [ "$lock_kind" = "mkdir" ]; then
     rm -rf "$LOCK_DIR" 2>/dev/null || true
   elif [ "$lock_kind" = "flock" ] || [ "$lock_kind" = "lockf" ]; then
-    exec 9>&- 2>/dev/null || true
+    exec 9>&-
   fi
   lock_kind=""
 }

scripts/install/test_install_sh.py

@@ -135,6 +135,27 @@ class InstallShTest(unittest.TestCase):
             )
             self.assertTrue(os.access(host_path, os.X_OK))
 
+    def test_noninteractive_install_warns_about_existing_npm_codex(self) -> None:
+        with tempfile.TemporaryDirectory() as temp_dir:
+            root = Path(temp_dir)
+            archive, checksum, metadata = create_package_release(root)
+            (root / "bin").mkdir()
+            write_executable(root / "bin" / "codex", "#!/usr/bin/env node\n")
+
+            result, _ = run_installer_in(
+                root,
+                VERSION,
+                metadata_json=metadata,
+                archive_path=archive,
+                checksum_path=checksum,
+                force_macos=True,
+            )
+
+            self.assertEqual(result.returncode, 0, result.stderr)
+            self.assertIn(
+                "Leaving the existing npm-managed Codex installed", result.stderr
+            )
+
     def test_releases_latest_installs_verified_package_by_default(self) -> None:
         with tempfile.TemporaryDirectory() as temp_dir:
             root = Path(temp_dir)