client_id is a required parameter in a OAuth access token request

> ##### 4.1.3.  Access Token Request
>
> [...]
>
>    client_id
>         REQUIRED, if the client is not authenticating with the
>         authorization server as described in Section 3.2.1.
>
> *-- https://datatracker.ietf.org/doc/html/rfc6749#section-4.1.3*

synapse/handlers/oidc.py

@@ -731,6 +731,7 @@ class OidcProvider:
         }
 
         args = {
+            "client_id": self._client_auth.client_id,
             "grant_type": "authorization_code",
             "code": code,
             "redirect_uri": self._callback_url,