fix(auth,web): fail onWaitInitState instead of hanging when the first auth state is an error

[chiliec]

Description

On Flutter web, Firebase.initializeApp() could hang forever (blank page until a manual reload) when the JS SDK's first onAuthStateChanged callback fires with an error — e.g. an IndexedDBLocalPersistence._openDb rejection during initial user restoration.

Auth.onWaitInitState() (awaited from FirebaseAuthWeb.registerWith → ensurePluginInitialized) had two defects on that path:

1. The error callback never completed the Completer, so await completer.future waited forever.
2. It dereferenced _changeController!, which is still null during plugin registration (nothing has subscribed to onAuthStateChanged yet), so the JS callback threw a null-check error and the original error was swallowed.

This PR:

• completes the completer with the error (guarded by isCompleted), so onWaitInitState() rejects instead of hanging;
• forwards the error to _changeController only when it exists (?.);
• unsubscribes the JS listener in a finally, on both the success and error paths;
• wraps the onWaitInitState() call in registerWith with the existing guardAuthExceptions, so Firebase.initializeApp() surfaces a regular FirebaseAuthException (FirebaseError → code/message conversion) instead of a raw JS object.

The happy path is unchanged: _initUser is still captured from the first next callback and onWaitInitState() still completes normally.

Related Issues

Fixes #18703

Tests

New packages/firebase_auth/firebase_auth_web/test/on_wait_init_state_test.dart (browser test) drives Auth.onWaitInitState() against a minimal JS stub of onAuthStateChanged:

• first auth state arrives → completes and unsubscribes;
• first auth state is an error → rejects with that error (with a 2 s timeout guard) and unsubscribes.

Run locally with flutter test --platform chrome (Flutter 3.47.2, chrome-headless-shell 153):

• on main the error case fails — TypeError: ... 'TypeErrorImpl' is not a subtype of type 'JSObject' from the _changeController! null check, and without that the future would never complete;
• with this change: on_wait_init_state_test.dart +2: All tests passed!, get_firebase_auth_exception_test.dart +2, auth_emulator_test.dart +11.

dart analyze on the package: No issues found!; dart format clean.

Not run here: the e2e example app / a real browser session with IndexedDB blocked.

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
• The analyzer (melos run analyze) does not report any problems on my PR.
• I read and followed the Flutter Style Guide.
• I signed the CLA.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.

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[dganzella]

I approve this, good stuff