Fix 4 npm dependency vulnerabilities

[Copilot]

Four vulnerabilities detected in transitive npm dependencies: two ReDoS (ajv, markdown-it), one DoS (diff), and one high-severity RCE (serialize-javascript).

Changes

• npm audit fix — upgraded ajv (6.12.6→6.14.0, 8.17.1→8.18.0) and markdown-it (14.1.0→14.1.1 via @vscode/vsce) to fix GHSA-2g4f-4pwh-qvx6 and GHSA-38c4-r59v-3vqw
• package.json overrides — forces safe versions of diff and serialize-javascript (transitive deps via mocha→@vscode/test-cli) to fix GHSA-73rr-hh4g-fpgx and GHSA-5c6j-r48x-rmvq without downgrading @vscode/test-cli from 0.0.12→0.0.11 (which npm audit fix --force would have done as a breaking change)

"overrides": {
  "diff": ">=8.0.3",
  "serialize-javascript": ">=7.0.3"
}

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