feat(firebase): tvOS support for firebase_app_check

[TheNoumanDev]

What does this PR do?

Adds federated firebase_app_check_tvos — Firebase App Check for Apple TV, built on the Firebase Apple SDK. Re-exports the firebase_app_check Dart API and ships the native tvOS pluginClass; depends on firebase_core_tvos.

DeviceCheck, App Attest, and Debug attestation providers work on tvOS 15+. reCAPTCHA is excluded — RecaptchaProvider is not part of the Firebase tvOS SDK; its Pigeon "recaptcha" case is kept as an unsupported-platform stub so registration/dispatch stay intact.

Package(s) touched: firebase_app_check_tvos (new)

How was it tested?

Native FirebaseAppCheck 12.15.0 initializes on tvOS. With the Debug provider, activate() issues a debug token and getToken() exchanges it with the App Check backend (exchangeDebugToken), returning a valid App Check token. Platform.operatingSystem == "tvos" / Platform.isIOS == true.

• Ran the package's example/ app
• Verified on tvOS simulator (26.2) — Debug-provider getToken round-trip succeeds against a live project
• Physical Apple TV — not verified. DeviceCheck / App Attest hardware attestation requires an App ID with the DeviceCheck/App Attest capability under a paid Apple Developer account; it can't be exercised on a free personal team. The providers compile and run on tvOS 15+, but genuine on-device attestation is pending that account. The Debug-provider path is fully verified. Would be great, if you can help with testing it on real device.
• dart analyze is clean for the package

Versioning & changelog

• version: set to 0.0.1 (new package)
• Matching ## 0.0.1 entry at the top of CHANGELOG.md
• No behaviour change (new package)
• Semver 0.x: initial 0.0.1

Checklist

• Only firebase_app_check_tvos files are touched
• No secrets, absolute local paths, or TODO/debug left
• README.md documents tvOS behaviour (reCAPTCHA excluded; DeviceCheck/App Attest need a paid account for real-device attestation; version-alignment note)
• Sibling-package constraint noted below

Notes for reviewers

• reCAPTCHA disabled on tvOS: RecaptchaProvider is iOS-only in the Firebase SDK. The "recaptcha" handler is kept as an unsupported-platform stub (never removed), so Pigeon dispatch doesn't break.
• Real-device attestation unverified: DeviceCheck/App Attest need a paid Apple Developer account + capability, so only the Debug provider is verifiable without it — and it's verified here on the simulator (token exchange round-trip).
• Version alignment (all Firebase _tvos leaf packages): the native Pigeon matches the firebase_core 4.11.x / firebase_core_platform_interface 7.1.0 train. Mixing firebase_core_tvos with a different firebase_core whose FirebaseOptions list differs can crash in CoreFirebaseOptions.fromList at launch (Dart options: init).

Relavant issue: #12