chore: pin vitest to 2.1.5

This works around vitest-dev/vitest#6992 being an issue with Vite 6

src/content/docs/workers/testing/vitest-integration/get-started/migrate-from-miniflare-2.mdx

@@ -30,7 +30,7 @@ First, you will need to uninstall the old environment and install the new pool.
 
 ```sh
 npm uninstall vitest-environment-miniflare
-npm install --save-dev --save-exact vitest@2.1.6
+npm install --save-dev --save-exact vitest@2.1.5
 npm install --save-dev @cloudflare/vitest-pool-workers
 ```