chore(test): enforce Vitest coverage thresholds in CI

[Olasunkanmi975]

##closed #751

Summary

Adds automated coverage enforcement to the test pipeline by configuring minimum Vitest coverage thresholds, integrating coverage reporting into CI, and documenting the project's coverage policy.

Changes

• Configured Vitest to use the V8 coverage provider with minimum thresholds:

  • Lines: 60%
  • Statements: 60%
  • Functions: 60%
  • Branches: 50%

• Updated the CI workflow to run coverage with JSON and LCOV reporters.

• Added LCOV upload for coverage trend tracking (using the repository's configured coverage service).

• Documented the coverage policy and local coverage workflow in CONTRIBUTING.md.

Testing

Verified:

• Coverage reports are generated in JSON and LCOV formats.
• Coverage threshold failures produce a non-zero exit code.
• CI reports coverage as part of the pull request checks.

Validation

npm test
npm run test:coverage
npm run lint

Notes

This change introduces coverage enforcement only. It does not modify application behavior or existing test logic.

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