fix(auth,ios): allow firebase_auth to compile under APPLICATION_EXTENSION_API_ONLY

[MohamedAbdallah-14]

Fixes #18232.

What's wrong

When firebase_auth is built into an iOS target with APPLICATION_EXTENSION_API_ONLY=YES (Share Extension, Action Extension, etc.), Xcode rejects four call sites in FLTFirebaseAuthPlugin.m:

'addApplicationDelegate:' is unavailable: not available on iOS (App Extension)
  FLTFirebaseAuthPlugin.m:155

'sharedApplication' is unavailable: not available on iOS (App Extension)
  FLTFirebaseAuthPlugin.m:903
  FLTFirebaseAuthPlugin.m:913
  FLTFirebaseAuthPlugin.m:925

The current workaround — setting APPLICATION_EXTENSION_API_ONLY = NO on the firebase_auth pod via a Podfile post-install hook — lets the build pass, but the resulting extension binary contains references to APIs that App Store review can flag.

Fix

Wrap the four sites in #if !TARGET_OS_EXTENSION:

1. [registrar addApplicationDelegate:instance] in +registerWithRegistrar: is gated; in extensions there is no application delegate to register against, so the openURL / scene-delegate paths below it are unreachable anyway.
2. The presentationAnchorForAuthorizationController: method already branched on TARGET_OS_OSX; this PR splits the iOS branch into !TARGET_OS_EXTENSION (existing UIScene/keyWindow logic) and a fallback that returns nil. Sign-in flows that need a presentation anchor cannot run from an extension context — the host app is the only place that can present them — so the method is not expected to be invoked from an extension build.

iOS and macOS host-app behaviour is unchanged.

Test plan

• Build a host iOS app with firebase_auth on Xcode 17 — confirm no regression in OAuth / Sign-in-with-Apple presentation flows.
• Build a Share Extension target that registers firebase_auth via FLTFirebaseAuthPlugin.register(with:) with APPLICATION_EXTENSION_API_ONLY=YES — confirm clean compile, no "not available on iOS (App Extension)" diagnostics.
• Build the macOS host app — confirm presentationAnchorForAuthorizationController: still returns the macOS key window.
• Verify the build succeeds for both Swift Package Manager and CocoaPods consumers.

I have not yet run all of these locally; opening as a draft so a maintainer or CI can validate the full matrix before this is marked ready.

Notes

• The macOS .m file at packages/firebase_auth/firebase_auth/macos/firebase_auth/Sources/firebase_auth/FLTFirebaseAuthPlugin.m is a symlink to the iOS source, so this single edit covers both platforms.
• I did not update CHANGELOG.md since flutterfire generates changelog entries from conventional commit subjects at release time via melos.

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[MohamedAbdallah-14]

@googlebot I signed it!