chore(deps-dev): bump @github/copilot in /test/harness (#655)

Bumps [@github/copilot](https://github.com/github/copilot-cli) from 0.0.420 to 0.0.421.
- [Release notes](https://github.com/github/copilot-cli/releases)
- [Changelog](https://github.com/github/copilot-cli/blob/main/changelog.md)
- [Commits](github/copilot-cli@v0.0.420...v0.0.421)

---
updated-dependencies:
- dependency-name: "@github/copilot"
  dependency-version: 0.0.421
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

test/harness/package-lock.json

@@ -11,7 +11,7 @@
     "test": "vitest run"
   },
   "devDependencies": {
-    "@github/copilot": "^0.0.420",
+    "@github/copilot": "^0.0.421",
     "@modelcontextprotocol/sdk": "^1.26.0",
     "@types/node": "^25.2.0",
     "openai": "^6.17.0",