build(deps): migrate to @cloudflare/vitest-plugin 1.x

@cloudflare/vitest-pool-workers was renamed to @cloudflare/vitest-plugin
for its v1 release; the old name stopped at 0.22.0 with pinned vulnerable
wrangler and miniflare versions. Vitest stays on 4: the plugin's peer
range is still ^4.1.0.

package.json

@@ -26,7 +26,7 @@
     "hono": "^4.12.30"
   },
   "devDependencies": {
-    "@cloudflare/vitest-pool-workers": "^0.17.0",
+    "@cloudflare/vitest-plugin": "^1.3.6",
     "vite": "^8.1.2",
     "vitest": "^4.1.9",
     "wrangler": "^4.112.0"