fix(deploy): declare firebase-admin in the generated Cloud Functions manifest

The root package.json had no firebase-admin entry, so the build resolved
its version to undefined and JSON.stringify dropped it from the generated
manifest. firebase-functions 6.x requires firebase-admin eagerly at
module load, so the deployed function crashed at cold start wherever
peers are not auto-installed (--legacy-peer-deps, yarn 1, pnpm).

firebase-functions moves to ^6.1.1, the first release whose peer range
accepts firebase-admin 13.

Author: armando-navarro