GHSA-9h47-pqcx-hjr4: Better Auth has insecure cryptographic defaults in oidcProvider: alg=none advertised and plain PKCE accepted by default

The legacy oidcProvider and mcp plugins exhibit two related defects in their OIDC discovery and authorize surfaces.

The discovery document advertises "none" in id_token_signing_alg_values_supported (and, for mcp, in resource_signing_alg_values_supported on the OAuth protected-resource metadata). Any relying party that performs algorithm negotiation from this metadata without pinning to a real signing algorithm may accept unsigned tokens.

PKCE plain is enabled by default. The runtime gate in the authorize handler accepts code_challenge_method=plain under this default, and a missing code_challenge_method parameter is silently downgraded to "plain" before the allowlist check. Discovery advertises code_challenge_methods_supported: ["S256"], contradicting the runtime acceptance of plain. RFC 9700 §2.1.1 (OAuth 2.1) explicitly forbids plain.