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/repository/api_key_repo.go

@@ -139,6 +139,7 @@ func (r *apiKeyRepository) GetByKeyForAuth(ctx context.Context, key string) (*se
 				group.FieldFallbackGroupIDOnInvalidRequest,
 				group.FieldModelRoutingEnabled,
 				group.FieldModelRouting,
+				group.FieldMcpXMLInject,
 			)
 		}).
 		Only(ctx)
@@ -428,6 +429,7 @@ func groupEntityToService(g *dbent.Group) *service.Group {
 		FallbackGroupIDOnInvalidRequest: g.FallbackGroupIDOnInvalidRequest,
 		ModelRouting:                    g.ModelRouting,
 		ModelRoutingEnabled:             g.ModelRoutingEnabled,
+		MCPXMLInject:                    g.McpXMLInject,
 		CreatedAt:                       g.CreatedAt,
 		UpdatedAt:                       g.UpdatedAt,
 	}