chore(tests): Upgrade vitest fixing the issues and increment test coverage

[gnbm]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Docs have been reviewed and added/updated if needed (for bug fixes/features)
• Build (npm run build) was run locally, and any changes were pushed
• Tests (npm test) were run locally and passed
• Prettier (npm run prettier) was run locally and passed

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe): Testing infrastructure improvements and coverage increase

What is the current behavior?

• Vitest was upgraded, which revealed a behavior change: vi.restoreAllMocks() no longer clears call history; one test began failing.
• Some utilities and network-related paths were not fully covered by tests.
• Overall coverage below 100%, and functions coverage could fail thresholds depending on changes.

GitHub Issue Number: N/A

What is the new behavior?

-Updated test tooling:
- vitest to ^4.0.4
- @vitest/coverage-v8 to ^4.0.4

• Stabilized tests for Vitest v4:

  • Added vi.clearAllMocks() in src/git.test.ts to isolate call history per test

• Extended test coverage:

  • Added src/download.mocked.test.ts to cover proxy agent usage, HEAD method for existence checks, and string URL handling without real network calls
  • Added tests in src/utils.test.ts:
    • Covered both branches of setTmpDirectory and validated registered process handlers (exit codes)
    • Exercised nodeVersionWarning edge cases, including the fallback when version parts are empty
    • Added a direct test for killChildren using fresh module state
    • Renamed onlyUnix tests for clarity (Windows returns string; Unix returns empty string)

• All tests passing locally

• Coverage:

  • 100% statements, branches, functions, and lines across the project

Does this introduce a breaking change?

• Yes
• No

Testing

• Local runs (Windows):
  • npm ci
  • npm test -- --run
  • Verified output shows all suites passing and full coverage:
    • Test Files: 6 passed
    • Tests: 54 passed
    • Coverage (V8): 100% across statements, branches, functions, lines
• Verified that no real network calls are made in mocked download tests
• Verified watch mode stability in Vitest v4

Other information

• New test file: src/download.mocked.test.ts
• Minor naming tweaks to improve test intent clarity in src/utils.test.ts
• This PR does not change runtime CLI behavior; it only improves testing accuracy, isolation, and coverage with the upgraded test runner.