GHSA-86j7-9j95-vpqj: Better Auth has stored XSS in the auth-server origin via javascript: redirect_uri in oidc-provider and mcp

The deprecated oidc-provider plugin registers OAuth clients without validating the scheme of their redirect_uris. An attacker stores a javascript: URI as a client redirect target, and the authorization server later returns that URI to the browser in the consent response. A consent page that navigates to the returned value then executes attacker JavaScript in the authorization-server origin, which exposes the victim’s session and enables account takeover. The mcp plugin wraps the same provider and carries the same defect, so MCP server deployments are affected as well; like oidc-provider, it is migrating to @better-auth/oauth-provider.