refactor: 统一自建 Tool Search — 移除 tool_reference/defer_loading 依赖，全 provider 通用

- 重命名 ExecuteTool → ExecuteExtraTool，作为一等工具始终可用
- ToolSearchTool 输出改为纯文本（区分 core/deferred），移除 tool_reference blocks
- 移除 modelSupportsToolReference() 及相关 GrowthBook 配置
- 移除 API 侧 defer_loading 字段和 tool search beta header 注入
- 简化 system prompt（工具使用指南从 ~120 行压缩到 ~10 行）
- extractDiscoveredToolNames 支持文本格式解析（向后兼容旧 session 的 tool_reference）
- 更新 promptEngineeringAudit 测试以匹配简化后的 prompt 结构

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/builtin-tools/src/tools/ExecuteTool/prompt.ts

@@ -1,10 +1,12 @@
 import { EXECUTE_TOOL_NAME } from './constants.js'
 
 export const DESCRIPTION =
-  'Execute a deferred tool by name with parameters. Use this after discovering a tool via ToolSearch.'
+  'ExecuteExtraTool — execute a deferred tool by name with parameters. This tool is always available. Use it after discovering a tool via ToolSearch.'
 
 export function getPrompt(): string {
-  return `Execute a deferred tool by name. This tool accepts a tool_name and params object, looks up the target tool in the global tool registry, and delegates execution to it.
+  return `ExecuteExtraTool — execute a deferred tool by name. This tool is always available in your tool list. You do NOT need to search for it.
+
+This tool accepts a tool_name and params object, looks up the target tool in the global tool registry, and delegates execution to it.
 
 Use this tool after discovering a deferred tool via ToolSearch. The tool_name must match the exact name returned by ToolSearch (e.g., "CronCreate", "mcp__server__action").