fix(test): scope root vitest config to src/__tests__ only

The Cloudflare Workers pool uses a virtual filesystem, so SDK tests
using readFileSync on the real filesystem would fail when picked up
by the root config. Scoping to src/__tests__/ keeps each package
running its own tests in the correct environment.

Author: DennisAlund

vitest.config.mts

@@ -10,4 +10,7 @@ export default defineConfig({
       },
     }),
   ],
+  test: {
+    include: ["src/__tests__/**/*.test.ts"],
+  },
 });