Open WebUI: Users denied by the OAuth role policy can still sign in via token exchange

## Summary Open WebUI's OAuth token exchange endpoint issues a session for a provider access token without running the OAuth role management that the normal OAuth login callback runs. A user whose provider roles the login callback would refuse, or would demote, could still obtain a working session at their existing role through this endpoint. ## Preconditions - `ENABLE_OAUTH_TOKEN_EXCHANGE=True`. It is disabled by default, so a default deployment is not affected. - `ENABLE_OAUTH_ROLE_MANAGEMENT=True` together with `OAUTH_ALLOWED_ROLES` or `OAUTH_ADMIN_ROLES`. Role management is off by default, and deployments not using it are not affected. - A valid, unexpired access token on the configured provider. - An Open WebUI account already linked to that provider subject, or an account with a matching email when `OAUTH_MERGE_ACCOUNTS_BY_EMAIL` is enabled. This endpoint never creates accounts, so a token for a subject with no existing account is rejected. ## Impact An admin who relies on OAuth role management expects a user to lose access, or lose admin, as soon as the identity provider stops reporting the required role. The login callback does enforce this on the next sign-in. Token exchange kept issuing sessions and never re-evaluated the role, so the user retained working access as their existing account at its existing role, including an admin role the provider had already revoked. The endpoint cannot create an account and cannot raise anyone's role, so this grants continued acces