fix: allow existing users to sign in and link via oauth

[ligong]

Purpose

This PR fixes a bug where existing users (e.g. registered via email/password) receive a `UserNotFoundAPIError` when trying to sign in using an OAuth provider (Google, GitHub, etc.) for the first time.

Problem

In _createUser (inside src/core/protocols/oauth/oauth_authentication.ts), if a user already exists, the function hits the
else block and returns null. Consequently, the calling function OAuthAuthentication treats this as a failed lookup and
returns UserNotFoundAPIError instead of auto-linking the new OAuth provider credentials (sub) to the existing user record.

Solution

Remove the else { return null } block from _createUser.
- If the user record exists, it is now correctly returned to the caller (OAuthAuthentication), allowing the provider
credentials to be linked and the user to log in successfully.
- If the user does not exist and allowOAuthAutoSignUp is false, userRecord remains null and is returned as null
(correctly rejecting sign-up).