OpaInfoFetcher missing/bad TLS config results in confusing error message

We moved from an on-prem Keycloak (16.0.1) to a managed keycloak (24.0.4) for a customer. It worked fine for basically all required products (Trino, Superset, Datahub etc.) but the OPA infofetcher was unhappy with just changing the endpoint and clientId / clientSecret.

user-info-fetcher 2024-12-05T11:52:15.473210Z  WARN stackable_opa_user_info_fetcher: Error while processing request error=failed to get user information from Keycloak error.sources=[failed to get access_token, http response 405 for "https://foo.bar.de/realms/foobar/protocol/openid-connect/token" with response body "{\"error\":\"HTTP 405 Method Not Allowed\",\"error_description\":\"For more on this error consult the server log at the debug level.\"}"] 

That was the error from the info fetcher. The get "access-token" call is 100% a POST HTTP call etc.
The managed Keycloak actually was complaining about (and receiving) a GET request.

This was very misleading, since the actual problem was missing the webPki trust.

    userInfo:
      backend:
        keycloak:
          hostname: "{{ .Values.keycloak }}"
          clientCredentialsSecret: "opa-infofetcher-secret"
          adminRealm: "foobar"
          userRealm: "foobar"
          tls:
            verification:
              server:
                caCert:
                  webPki: {}

Since Trust/TLS this is a very common configuration mistake, i think the error messages should reflect this better.

Edit: Since this may depend on the (managed) keycloak server and setup, I did not check if i could reproduce this with other setups.

No file or test is named; start by locating the OpaInfoFetcher access-token request and its TLS/error handling. Reproduce with the shown Keycloak configuration, then verify that missing or incorrect webPki trust produces an error that identifies the TLS problem rather than only the HTTP 405.