[Bug] [iOS] App delegate swizzler has no UIScene equivalent — cold-launch & universal links miss Analytics/Dynamic Links on scene-based apps

### Environment
- Platform: iOS (scene-based lifecycle, UIWindowSceneDelegate)
- Firebase C++ SDK version: (also consumed via Firebase Unity SDK 12.10.1)
- Installed via: Unity (FirebaseCppApp / FirebaseCppAnalytics), with FirebaseAnalytics/FirebaseCore pods 11.14.0, GoogleUtilities 8.1.2
- Relevant products: Analytics (campaign attribution)

### Problem Description
The C++ SDK's iOS app-delegate swizzler (app/src/invites/ios/invites_ios_startup.mm, hooking installed via +load → RunOnAppDelegateClasses → HookAppDelegateMethods) only replaces UIApplicationDelegate methods:
- application:openURL:options:
- application:openURL:sourceApplication:annotation:
- application:continueUserActivity:restorationHandler:
- applicationDidBecomeActive:

There is no UISceneDelegate/UIWIndowSceneDelegate handling anywhere in the C++ SDK (looking at unity-v12.10.1 and v13.10.0)

On modern scene-based apps (UIWindowSceneDelegate), URL/NSUserActivity delivery moves off the app delegate onto the scene delegate. The C++ SDK adds nothing on top. As a result these link-delivery paths reach neither the C++ Dynamic Links machinery nor Analytics:
- scene:willConnectToSession:options: — cold-launch deep links (connectionOptions.URLContexts) and cold-launch universal links (connectionOptions.userActivities)
- scene:openURLContexts: — deep links delivered while the scene is already connected
- scene:continueUserActivity: — universal links delivered while the scene is already connected

**Impact:** on scene-based iOS apps, cold-launch deep/universal links and connected-scene universal links are silently dropped by the SDK.

#### Steps to reproduce:
1. Scene-based iOS app (Unity export uses UnityScene : UIWindowSceneDelegate in their latest Editor versions).
2. Register a Dynamic Link / campaign URL.
3. Cold-launch the app via a universal link → delivered to scene:willConnectToSession:options:.
4. Observe: no firebase::/GUL frame in the stack for that delivery; no Dynamic Link resolution; no Analytics campaign event. (Same for scene:continueUserActivity: or scene:openURLContexts: while connected.)

#### Question for maintainers:
- Is scene-delegate support planned for the C++ SDK's swizzler?

#### Workaround we're using (and would like validated):
We swizzle only the three uncovered scene methods on the scene delegate and forward to [FIRAnalytics handleOpenURL:] / [FIRAnalytics handleUserActivity:], chaining to the original implementation so the engine's own deep-link handling is preserved. Is this the recommended approach, and is there any double-handling risk if the SDK later completes scene coverage?

app/src/invites/ios/invites_ios_startup.mm から始め、RunOnAppDelegateClasses と HookAppDelegateMethods を、記載されている UIApplicationDelegate フックと併せて読みます。issue で説明されている scene delegate の配信経路を追跡し、元の実装を維持しながら Dynamic Links と Analytics にカバレッジを到達させる方法を判断します。完了の条件は、報告されている cold-launch と接続済みシーンの URL および user-activity の経路が、暗黙のうちに破棄されなくなることです。

索引モデルが issue の本文から書いたものです。