Gitea WebAuthn bypass during OAuth and OIDC sign-in

Gitea's OAuth2 and OpenID Connect sign-in paths do not require a WebAuthn challenge when WebAuthn is the account's only configured second factor. A party able to authenticate through the affected external identity flow can obtain a full session without the passkey verification enforced during password login. One affected path can also persist an external identity link, extending the compromise beyond the initial session; accounts with TOTP configured are outside the reported WebAuthn-only scenario.