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/ent/migrate/schema.go

@@ -229,6 +229,7 @@ var (
 		{Name: "fallback_group_id_on_invalid_request", Type: field.TypeInt64, Nullable: true},
 		{Name: "model_routing", Type: field.TypeJSON, Nullable: true, SchemaType: map[string]string{"postgres": "jsonb"}},
 		{Name: "model_routing_enabled", Type: field.TypeBool, Default: false},
+		{Name: "mcp_xml_inject", Type: field.TypeBool, Default: true},
 	}
 	// GroupsTable holds the schema information for the "groups" table.
 	GroupsTable = &schema.Table{