feat(group): 添加 MCP XML 注入开关

- Group 新增 mcp_xml_inject 字段，控制 Antigravity 平台的 MCP XML 协议注入
- 默认启用，可在分组设置中关闭
- 修复 GetByKeyForAuth 遗漏查询 mcp_xml_inject 字段导致认证缓存值始终为 false 的问题

backend/internal/service/api_key_auth_cache_impl.go

@@ -224,6 +224,7 @@ func (s *APIKeyService) snapshotFromAPIKey(apiKey *APIKey) *APIKeyAuthSnapshot {
 			FallbackGroupIDOnInvalidRequest: apiKey.Group.FallbackGroupIDOnInvalidRequest,
 			ModelRouting:                    apiKey.Group.ModelRouting,
 			ModelRoutingEnabled:             apiKey.Group.ModelRoutingEnabled,
+			MCPXMLInject:                    apiKey.Group.MCPXMLInject,
 		}
 	}
 	return snapshot
@@ -269,6 +270,7 @@ func (s *APIKeyService) snapshotToAPIKey(key string, snapshot *APIKeyAuthSnapsho
 			FallbackGroupIDOnInvalidRequest: snapshot.Group.FallbackGroupIDOnInvalidRequest,
 			ModelRouting:                    snapshot.Group.ModelRouting,
 			ModelRoutingEnabled:             snapshot.Group.ModelRoutingEnabled,
+			MCPXMLInject:                    snapshot.Group.MCPXMLInject,
 		}
 	}
 	return apiKey