Sign in with OAuth2 on Windows and Linux through a loopback token callback in the Flutter SDK #2094

Tracks PR #2093.

Plan

Goal: Make OAuth2 sign-in work in the generated Flutter SDK on Windows and Linux, where flutter_web_auth_2 only accepts an http://localhost:{port} callback and createOAuth2Session never returns key/secret to a custom success URL.

What changes:

• templates/flutter/lib/src/client_io.dart.twig: on Windows and Linux, webAuth uses the given success URL (or a free loopback port, http://localhost:<port>/) as the callback, sets success and (unless given) failure to it, and for createOAuth2Session opens /account/tokens/oauth2/{provider} and exchanges the returned userId/secret via POST /account/sessions/token. Android, iOS and macOS keep the appwrite-callback-<projectId> flow; saving that session now waits for an in-flight init() and awaits the cookie write.
• templates/flutter/test/src/client_io_test.dart.twig (new) and src/SDK/Language/Flutter.php: generate a test covering loopback sign-in, given success/failure URLs and error callbacks on Linux and Windows, plus the Android callback flow.

Remaining steps: None beyond the linked PR. Projects still need a Web platform for localhost so the server accepts the loopback success URL, which is worth mentioning in Flutter SDK docs.

Done when:

• fix(flutter): sign in with OAuth2 on Windows and Linux through a loopback token callback #2093 is merged and the generated Flutter SDK test suite passes.
• The fix ships in a Flutter SDK release.