refactor: initializeApp() with optional params, add multi-app support, service caching, and auto project ID discovery

[demolaf]

Changes

1. No App Registry

• Before: Had to manually track app instances
• Now: FirebaseApp.getApp('myApp') works like Node.js

2. Services Weren't Cached

• Before: Auth(app) created a new instance every time
• Now: Returns the same instance (singleton pattern per app)

3. Manual Project ID Required

• Before: FirebaseAdminApp.initializeApp(projectId, credential)
• Now: Auto-discovers from credentials, env vars, or metadata server

4. No Proper Cleanup

• Before: Services didn't clean up when app closed
• Now: app.close() notifies all services to cleanup

5. Manual Emulator Setup

• Before: Had to call app.useEmulator() manually
• Now: Auto-detects from FIREBASE_*_EMULATOR_HOST env variables

6. No Named Apps

• Before: Could only have one app
• Now: initializeApp(options, name: 'secondary') like Node.js

Key Classes Introduced

• ProjectIdProvider - Handles project ID discovery from multiple
  sources
• AppRegistry - Manages app instances by name with lifecycle hooks
• FirebaseApp - New app class with Node.js-compatible API
• FirebaseService - Interface for proper service lifecycle
  management
• AppOptions - Cleaner way to configure apps
• AppException - Proper error handling for app-level errors
• Environment - Handles environment detection and emulator config

Example Scripts

• example/run_with_emulator.sh - Run example against emulators
• example/run_with_prod.sh - Run example against production

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Coverage Report

✅ Coverage 54.93% meets 40% threshold

Coverage: 54.93%
Lines Covered: 2347/4273

Minimum threshold: 40%

[Lyokone]

Minor changes