fix(auth): ensure PigeonAuthCredential is passed back to Dart side within try/catch

[russellwheatley]

Description

Whilst testing this code, I observed a few issues:

TwitterAuthProvider twitterProvider =
    TwitterAuthProvider();
try {
  await FirebaseAuth.instance.currentUser!
      .linkWithProvider(twitterProvider);
} on FirebaseAuthException catch (e) {
  if (e.code == 'credential-already-in-use') {
    print(
        'The account already exists for that email. - ${e.credential}');
    await FirebaseAuth.instance.signOut();
    final cred = await FirebaseAuth.instance
        .signInWithCredential(e.credential!);
  }
}

1. We were not passing back the AuthCredential when present on the exception which was an issue for both andriod and iOS. Fixed here.
2. We were passing back a dictionary of AuthCredential on iOS side, we needed to pass back the PigeonAuthCredential. Fixed here.
3. We were passing a generic "sign-in-failed" exception and not using the code from the error. Fixed here.
4. Upon using signInWithCredential after linkWithProvider failed with this exception code credential-already-in-use, we were receiving an exception code for signInWithCredential that was network error code (i.e. the number "400"), I have updated to check if the error code is a number and responded with the underlying error code. Fixed here.

Related Issues

fixes #11164
fixes #11686

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[suysoftware]

/Users/suy/.pub-cache/hosted/pub.dev/firebase_auth-4.11.0/macos/Classes/PigeonParser.m:5:9: fatal error: 'PigeonParser.h' file not found
#import "PigeonParser.h"
^~~~~~~~~~~~~~~~

[sunmeetsingh2606]

/Users/suy/.pub-cache/hosted/pub.dev/firebase_auth-4.11.0/macos/Classes/PigeonParser.m:5:9: fatal error: 'PigeonParser.h' file not found #import "PigeonParser.h" ^~~~~~~~~~~~~~~~

Facing same issue with the latest version of firebase_auth (4.11.0)