fix(ext/build): exclude test files from webpack tsc compile

Slice 4 spec review caught: router.test.ts's narrow chrome.* shim
triggered 4 TS errors in webpack's ts-loader pass during production
builds (partial mocks don't match the full chrome.* type surface).

Plan's verbatim test body assumes tests aren't part of the build
compile. Add src/**/__tests__/** to tsconfig exclude — tests still
compile under Vitest's independent ts pipeline (42/42 passing).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

extension/tsconfig.json

@@ -15,5 +15,5 @@
     "baseUrl": "."
   },
   "include": ["src/**/*"],
-  "exclude": ["node_modules", "dist", "wasm"]
+  "exclude": ["node_modules", "dist", "wasm", "src/**/__tests__/**"]
 }