feat(pi-web-search): 统一 web_search/web_fetch 并由主模型路由搜索厂商

7 个按厂商命名的搜索工具收敛为 2 个统一工具，schema 与提示词不再暴露
provider 概念：web_search 由当前主模型按内置规则在 Tavily/Exa/Keenable
间路由（会话内 LRU 缓存，失败按可用性降级），web_fetch 固定走 Exa 并
静默降级。三家均改为纯 fetch 直连——Exa 直接对 mcp.exa.ai 发 JSON-RPC
并缓存会话，彻底移除 pi-mcp-adapter、@tavily/pi-extension 与
@keenable/pi-search 依赖。

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tests/extension-load.test.ts

@@ -35,6 +35,11 @@ test("all package extensions load together without global registration conflicts
 		packageJson.pi.extensions.includes("./extensions/codegraph.ts"),
 		"the bundle must expose the native codegraph_explore tool",
 	);
+	assert.ok(
+		packageJson.pi.extensions.includes("./pi-web-search/index.ts"),
+		"the bundle must load the unified pi-web-search tools",
+	);
+	assert.equal(packageJson.dependencies["pi-web-search"], "file:./pi-web-search");
 	assert.equal(
 		packageJson.pi.extensions.includes("./extensions/plugin-init.ts"),
 		false,