docs(gotrue): require onError handler in onAuthStateChange

[grdsdev]

Summary

• Updates onAuthStateChange doc comment in GoTrueClient with an explicit warning that onError is required, plus a corrected usage example
• Fixes the example app which was missing onError and would crash when the device had no connectivity

Background

Closes #1281.

Network errors (e.g. a token refresh while offline) are emitted as stream errors on onAuthStateChange. Without an onError handler, Dart rethrows them as unhandled zone exceptions, crashing the app. The SDK's own internal listener in supabase_auth.dart already handles this correctly — the gap was in documentation and the example app.

A companion PR in the reference docs repo updates all code examples on supabase.com: supabase/supabase#44946

Changes

packages/gotrue/lib/src/gotrue_client.dart

• Rewrote the onAuthStateChange doc comment to warn that onError is required
• Updated the inline code example to show correct usage with onError

packages/supabase_flutter/example/lib/main.dart

• Added onError handler to the .listen() call that was missing it

Test plan

• Confirm example app no longer crashes when device goes offline during a token refresh
• Confirm doc comment renders correctly in IDE tooling

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