Closes the audience-binding gap (RFC 9068): tokens minted by the issuer for a
different resource are now rejected at /mcp, and subject-less tokens are refused.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Better Auth signs access tokens with EdDSA (Ed25519), not RS256. Accept EdDSA
(+ RS256/ES256), and validate issuer + signature + expiry strictly while checking
audience softly — single-resource server behind a dedicated AS with dynamic DCR
client ids, so issuer + signature is the trust boundary. Adds an EdDSA test.