chore(deps-dev): bump openai from 6.17.0 to 6.21.0 in /test/harness

Bumps [openai](https://github.com/openai/openai-node) from 6.17.0 to 6.21.0.
- [Release notes](https://github.com/openai/openai-node/releases)
- [Changelog](https://github.com/openai/openai-node/blob/master/CHANGELOG.md)
- [Commits](openai/openai-node@v6.17.0...v6.21.0)

---
updated-dependencies:
- dependency-name: openai
  dependency-version: 6.21.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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

Author: dependabot[bot]

test/harness/package-lock.json

@@ -13,7 +13,7 @@
   "devDependencies": {
     "@github/copilot": "^0.0.403",
     "@types/node": "^25.2.0",
-    "openai": "^6.17.0",
+    "openai": "^6.21.0",
     "tsx": "^4.21.0",
     "typescript": "^5.9.3",
     "vitest": "^4.0.18",