fix: resolve CI failures — gofmt, unused functions, missing test helpers

- Run gofmt on user schema, config test, group handler
- Remove unused mergeGroupIDs function
- Restore shared test helpers (newJSONResponse, queuedHTTPUpstream)
  that were in deleted Sora test file

backend/internal/service/openai_oauth_service.go

@@ -374,20 +374,6 @@ func (s *OpenAIOAuthService) Stop() {
 	s.sessionStore.Stop()
 }
 
-func (s *OpenAIOAuthService) resolveProxyURL(ctx context.Context, proxyID *int64) (string, error) {
-	if proxyID == nil {
-		return "", nil
-	}
-	proxy, err := s.proxyRepo.GetByID(ctx, *proxyID)
-	if err != nil {
-		return "", infraerrors.Newf(http.StatusBadRequest, "OPENAI_OAUTH_PROXY_NOT_FOUND", "proxy not found: %v", err)
-	}
-	if proxy == nil {
-		return "", nil
-	}
-	return proxy.URL(), nil
-}
-
 func normalizeOpenAIOAuthPlatform(platform string) string {
 	return openai.OAuthPlatformOpenAI
 }