fix(auth, iOS): add detailed error handling for Apple authorization failures

[SelaseKay]

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Fixes #18413

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[bjrochem72]

Thanks for picking this up so quickly — the native side is exactly what I was hoping for.

Two things I noticed when tracing what happens to the new error on the Dart side, one of which I think is worth fixing before merge:

1. The Underlying: label gets the message truncated in Dart.

In firebase_auth_platform_interface/lib/src/method_channel/utils/exception.dart, the Pigeon path does:

var parsedMessage = platformException.message?.split(': ').last;

The new format includes the literal ", Underlying: Domain=%@ Code=%ld", so whenever an underlying error is present, everything before that ": " is discarded. For example:

Built natively:
The operation couldn't be completed. (com.apple.AuthenticationServices.AuthorizationError error 1000.) (Domain=com.apple.AuthenticationServices.AuthorizationError Code=1000, Underlying: Domain=AKAuthenticationError Code=-7026)

Received in Dart:
Domain=AKAuthenticationError Code=-7026)

— the localizedDescription and the primary Domain/Code are lost, which is the exact case (AKAuthenticationError −7026) that motivated the issue. The no-underlying case is fine, since Apple's synthesized descriptions don't contain ": ".

Minimal fix: drop the colon from the label, e.g.

[NSString stringWithFormat:@", Underlying Domain=%@ Code=%ld", ...]

2. The structured details dictionary doesn't currently reach app code.

platformExceptionToFirebaseAuthException only reads authCredential and email from details, and FirebaseAuthException has no field to carry the rest, so nativeErrorDomain / nativeErrorCode / underlyingNativeErrorDomain / underlyingNativeErrorCode are dropped during conversion. Totally fine by me if the message stays the single source (that's parseable once point 1 is fixed) — just flagging it so the keys aren't assumed to be reachable from Dart.

Happy to test the branch against the device states from the original report once it's updated.

[SelaseKay]

Good catch @bjrochem72. I've updated the PR. Will wait for your feedback on the test.

[bjrochem72]

Tested the updated branch on device (iOS 26.4 simulator, no Apple ID signed in — the AuthorizationError 1000 case from the report). Confirming the truncation fix in 5a0c012d works: Dart now receives

[firebase_auth/unknown] The operation couldn't be completed. (com.apple.AuthenticationServices.AuthorizationError error 1000.) (Domain=com.apple.AuthenticationServices.AuthorizationError Code=1000)

— the primary Domain/Code survive the platform-interface message.split(': ').last path intact, where before this PR it was just An unknown error has occurred. The colon-free , Underlying Domain=… Code=… label is the right fix for the underlying-error case; I wasn't able to force AKAuthenticationError -7026 deterministically on device, but by inspection nothing before that label can be truncated now, so 👍.

Note on my harness: I couldn't pull the auth_18413 branch directly because its Dart layer references FirebasePlugin/pluginConstants from unreleased firebase_core master. I tested by applying just this PR's FLTFirebaseAuthPlugin.m on top of the released firebase_auth 6.5.4 — so this exercises exactly the native change.

On "the test": I only see FLTFirebaseAuthPlugin.m in the diff — if you pushed a test it may not have synced. Since the details dictionary isn't surfaced to Dart, an ObjC-level unit test over convertAppleAuthorizationErrorToFlutterError: (assert the formatted message and the nativeErrorDomain/nativeErrorCode/underlying keys, with and without an NSUnderlyingError) would cover it best. Happy to review once it's up.

Unrelated: the web (tests) red looks like infra/flake — this is an iOS-only ObjC change.