When a GitHub user's profile email is null (hidden/private), the OAuth callback fails with HTTP 400 because `GitHubRawUserInfo` validates `email` as a required non-null string. Even after the type was relaxed to `NotRequired[str | None]` in #33882, the flow still raises a `ValueError` when no email can be resolved, blocking sign-in entirely.
This PR improves the email resolution strategy so that users with private GitHub emails can still sign in.
