fix: resolve retired/preview gemini model routing and test configuration errors

internal/server/models_config.go

@@ -49,6 +49,7 @@ func (s *Server) handleGetModels(c *gin.Context) {
 
 	var result []gin.H
 	s.registryMu.RLock()
+	defer s.registryMu.RUnlock()
 	if s.registry != nil {
 		for pID, pInfo := range s.registry.Providers {
 			proxyProvider, allowed := allowedRegistryProviders[pID]
@@ -208,6 +209,7 @@ func (s *Server) handleUpdateModel(c *gin.Context) {
 			}
 		}
 	}
+	s.registryMu.RUnlock()
 
 	_, err := s.database.Exec(`
 		INSERT INTO model_configs (id, provider_id, enabled, prompt_cost_per_m, completion_cost_per_m, cache_read_cost_per_m, cache_write_cost_per_m, mapping)