The OIDC Dynamic Client Registration spec details the id_token_signed_response_alg parameter. The alg value of this parameter should be used to sign the id_token.
This is needed to pass the oidcc-idtoken-unsigned test from the certification basic plan.

oidc-core §3.1.3.7

The alg value SHOULD be the default of RS256 or the algorithm sent by the Client in the id_token_signed_response_alg parameter during Registration.

oidc-registration §2

id_token_signed_response_alg
OPTIONAL. JWS alg algorithm [JWA] REQUIRED for signing the ID Token issued to this Client. The value none MUST NOT be used as the ID Token alg value unless the Client uses only Response Types that return no ID Token from the Authorization Endpoint (such as when only using the Authorization Code Flow). The default, if omitted, is RS256. The public key for validating the signature is provided by retrieving the JWK Set referenced by the jwks_uri element from OpenID Connect Discovery 1.0 [OpenID.Discovery].

oidc-discovery §3

id_token_signing_alg_values_supported
REQUIRED. JSON array containing a list of the JWS signing algorithms (alg values) supported by the OP for the ID Token to encode the Claims in a JWT [JWT]. The algorithm RS256 MUST be included. The value none MAY be supported but MUST NOT be used unless the Response Type used returns no ID Token from the Authorization Endpoint (such as when using the Authorization Code Flow).