chore: validate release Firebase config

[jjoonleo]

Summary

• add Gradle validation for the release Android google-services.json package name
• document the release Firebase config source, local validation command, and fingerprint handoff
• add a [Release] Configure release Firebase/Google services file #451 implementation plan under plans/

Closes #451
Refs #467

Verification

• flutter pub get
• dart run build_runner build --delete-conflicting-outputs
• gradle :app:validateAndroidGoogleServices with a temporary matching android/app/src/release/google-services.json: passed
• gradle :app:validateAndroidGoogleServices with a temporary mismatched package: failed with the expected package-name error
• flutter analyze: passed

Unblocks

• contributes the Firebase config prerequisite for [Release] Build a signed release Android App Bundle #452
• documents the Firebase fingerprint follow-up needed by [Release] Verify Play App Signing and certificate fingerprints #454