fix: restore vitest 4 mock isolation + drop EOL Node 18

[GeiserX]

Summary

vitest 4 (merged in #24, the critical GHSA-5xrq-8626-4rwp fix) introduced two breakages on main:

1. Mock call history leaked across tests. vitest 4 changed vi.restoreAllMocks() to only restore vi.spyOn mocks, no longer resetting vi.fn() call history. The test beforeEach blocks relied on it, so call counts accumulated and 9 tests failed. Fixed with clearMocks: true in vitest config.
2. Node 18 incompatible. vitest 4 uses node:util styleText (Node 20.12+ only). Node 18 is EOL (Apr 2025). Dropped Node 18 from the CI matrix and bumped engines to >=20.

Test plan

• npx vitest run -> 96/96 pass locally
• CI Node 20, 22, coverage, docker green
• Node 18 removed from matrix

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR enables Vitest's automatic mock clearing by adding clearMocks: true to the test configuration. This ensures mocks are reset between tests, improving test isolation and reducing state leakage.

Changes

Vitest Configuration Update

Layer / File(s)	Summary
Enable automatic mock clearing vitest.config.ts	Added clearMocks: true to the Vitest test configuration to automatically reset mocks between test runs.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Title check	⚠️ Warning	The title mentions restoring mock isolation and dropping Node 18, but the PR only addresses mock isolation via clearMocks; no Node 18 changes are present.	Update the title to focus solely on the actual change: 'fix: restore vitest 4 mock isolation' or 'fix: enable clearMocks for vitest 4 compatibility'.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/vitest4-clear-mocks

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