CVE-2026-53517: Better Auth: OAuth refresh-token rotation forks the token family on concurrent redemption

The OAuth provider’s POST /oauth2/token endpoint, on the refresh_token grant, performs a non-atomic read / validate / revoke / mint sequence on the oauthRefreshToken row. Two concurrent requests presenting the same parent refresh token both pass the revocation check before either revoke completes, so each mints a fresh refresh token. The replay-detection branch only fires when revoked is already truthy at read time, which is exactly the state concurrent attackers race past. The result is a forked refresh-token family from a single parent token.