Update @cloudflare/vitest-pool-workers requirement in /packages/worker

Updates the requirements on [@cloudflare/vitest-pool-workers](https://github.com/cloudflare/workers-sdk/tree/HEAD/packages/vitest-pool-workers) to permit the latest version.
- [Release notes](https://github.com/cloudflare/workers-sdk/releases)
- [Changelog](https://github.com/cloudflare/workers-sdk/blob/main/packages/vitest-pool-workers/CHANGELOG.md)
- [Commits](https://github.com/cloudflare/workers-sdk/commits/@cloudflare/vitest-pool-workers@0.12.18/packages/vitest-pool-workers)

---
updated-dependencies:
- dependency-name: "@cloudflare/vitest-pool-workers"
  dependency-version: 0.12.18
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

packages/worker/package.json

@@ -20,7 +20,7 @@
 	},
 	"devDependencies": {
 		"@babel/preset-env": "^7.29.0",
-		"@cloudflare/vitest-pool-workers": "^0.12.14",
+		"@cloudflare/vitest-pool-workers": "^0.12.18",
 		"@cloudflare/workers-types": "^4.20260227.0",
 		"@eslint/js": "^10.0.1",
 		"@typescript-eslint/eslint-plugin": "^8.56.0",