feat(firebase): tvOS support for Core, Auth, Firestore, Storage, Messaging

[MAUstaoglu]

Summary

Adds federated *_tvos implementations bringing Firebase to Apple TV via the
flutter-tvos toolchain, built on the Firebase Apple SDK (which has supported
tvOS since 8.9.0). This closes the gap where the FlutterFire packages declare
ios/macos but not tvos, even though the underlying native SDK runs on tvOS.

Package	tvOS support
firebase_core_tvos	Full
cloud_firestore_tvos	Full
firebase_storage_tvos	Full
firebase_auth_tvos	Anonymous, email/password, email-link, Sign in with Apple, custom-token. MFA, phone, browser-OAuth providers and reCAPTCHA are absent from the Firebase tvOS SDK and return unsupported-platform.
firebase_messaging_tvos	FCM token, topic subscription, permission request. Rich-notification payload (onMessage) and tapped-notification/interaction APIs are unavailable on tvOS.

Each package re-exports the upstream Dart API and ships the native tvOS
pluginClass; the four leaf packages depend on firebase_core_tvos.

Runtime verification

Verified against a live Firebase project on both the tvOS simulator (JIT) and
a physical Apple TV (tvOS 26.5, AOT/profile):

• firebase_core — initializeApp succeeds
• firebase_auth — anonymous sign-in returns a real user
• cloud_firestore — document write + read-back round-trip
• firebase_storage — executes; returns a normal Storage error when no bucket is provisioned
• firebase_messaging — executes; returns apns-token-not-set (APNs needs a push-enabled profile)

Platform.operatingSystem == "tvos" and Platform.isIOS == true confirmed under AOT.

Two issues that only surfaced at runtime were fixed:

• firebase_auth_tvos aborted at plugin registration because the generated Pigeon
  setup asserts every multi-factor selector exists; the tvOS-disabled methods are
  now kept as unsupported-platform stubs rather than removed.
• firebase_auth_tvos + firebase_messaging_tvos in the same app failed to compile
  (-[FIRAuth canHandleNotification:], unavailable on tvOS); now guarded.

Notes

Each package includes a PORTING_REPORT.md documenting exactly which native APIs
were adapted or disabled and why. These are 0.0.1 initial releases; the intended
publish order is firebase_core_tvos first, then the four leaf packages.