ci: split coverage into separate workflow

[sobeaa]

Summary

This pull request updates how code coverage is handled in the workflow by moving coverage reporting into a dedicated workflow file. The main change is to separate the coverage upload step from the general code quality checks, making the CI configuration more modular and easier to maintain.

Workflow configuration changes:

• Created a new workflow file, .github/workflows/coverage.yml, which is responsible for running tests and uploading coverage reports using Codecov when changes are pushed to the main branch.
• Removed the coverage upload step from the existing .github/workflows/code-quality.yml workflow, so it no longer handles coverage reporting.

Related issue

Closes #12

Type of change

• Bug fix
• Feature
• Refactor
• Documentation
• Test improvement

Validation

• npm run format:check
• npm run lint
• npm run typecheck
• npm run knip
• npm test
• npm run build

Checklist

• PR targets main.
• The change is linked to an issue, or an issue is not required.
• The solution is minimal and intentional.
• Tests were added or updated for behavior changes.
• Documentation was updated where relevant.