Migrate extensions/copilot/chat-lib to Vitest v4.1.7

[Copilot]

extensions/copilot/chat-lib was still pinned to Vitest v3. This change upgrades the package to Vitest v4 and refreshes the lockfile so dependency resolution is consistent for consumers and CI.

• Description
  • Dependency upgrade
    • Updated devDependencies.vitest in extensions/copilot/chat-lib/package.json from ^3.0.5 to 4.1.7 (exact pin).
  • Resolved dependency graph
    • Regenerated extensions/copilot/chat-lib/package-lock.json to capture the Vitest v4.1.7 ecosystem updates.
  • Scope
    • No source/runtime logic changes in chat-lib; this PR is dependency-only.

// extensions/copilot/chat-lib/package.json
{
  "devDependencies": {
    "vitest": "4.1.7"
  }
}

Original prompt

Under extensions/copilot/chat-lib, migrate vitest to v4. Create a PR.

Created from VS Code.

[rzhao271]

@copilot Pin vitest to 4.1.7 and try again

[Copilot]

@copilot Pin vitest to 4.1.7 and try again

Updated in commit 6c5ec0e1: extensions/copilot/chat-lib now pins vitest to exact 4.1.7, and package-lock.json was refreshed accordingly.