Fix Cloud Build failure by adding Linux rolldown bindings

The release build in Cloud Build (Linux) was failing because `npm ci` could not find the `@rolldown/binding-linux-x64-gnu` native binding. This is a known issue when lockfiles are generated on non-Linux OSes.

Explicitly adding it to `optionalDependencies` ensures it's included in `package-lock.json` and correctly installed by `npm ci` on Linux, while being safely skipped on other platforms.

Author: taeold

package-lock.json

@@ -338,6 +338,9 @@
     "express": "^4.21.0",
     "protobufjs": "^7.2.2"
   },
+  "optionalDependencies": {
+    "@rolldown/binding-linux-x64-gnu": "1.0.0-beta.45"
+  },
   "devDependencies": {
     "@eslint/eslintrc": "^3.3.1",
     "@firebase/api-documenter": "^0.2.0",