feat: enhance issuer URL validation in OAuth server metadata (#2164)

## Summary
- Add trailing slash removal to prevent double slashes in endpoint URLs
- Update validation comment to clarify purpose

Author: cemalkilic

internal/api/oauthserver/handlers.go

@@ -233,11 +233,14 @@ type OAuthServerMetadataResponse struct {
 func (s *Server) OAuthServerMetadata(w http.ResponseWriter, r *http.Request) error {
 	issuer := s.config.JWT.Issuer
 
-	// TODO(cemal) :: Remove this check when we have the config validation in place
+	// Basic issuer validation - empty issuer would create broken URLs
 	if issuer == "" {
 		return apierrors.NewInternalServerError("Issuer is not set")
 	}
 
+	// Ensure issuer doesn't end with a slash to avoid double slashes in URLs
+	issuer = strings.TrimSuffix(issuer, "/")
+
 	response := OAuthServerMetadataResponse{
 		Issuer:                            issuer,
 		AuthorizationEndpoint:             fmt.Sprintf("%s/oauth/authorize", issuer),