fix(openid): log the token endpoint auth method on exchange failure

Makes the next failed token exchange diagnosable from the Vikunja side without
having to correlate against the provider's logs.

Author: kolaente

pkg/modules/auth/openid/openid.go

@@ -620,6 +620,8 @@ func exchangeOidcTokens(cb *Callback, providerKey string) (*Provider, *oauth2.To
 	// Parse the access & ID token
 	oauth2Token, err := provider.Oauth2Config.Exchange(context.Background(), cb.Code)
 	if err != nil {
+		log.Debugf("Token exchange failed for provider %s using token_endpoint_auth_method %s", provider.Key, authStyleName(provider.Oauth2Config.Endpoint.AuthStyle))
+
 		var rerr *oauth2.RetrieveError
 		if errors.As(err, &rerr) {