Enabled running mixed Vitest and Karma tests with merged coverage report

[martnpaneq]

🚀 Summary

Enabled running mixed Vitest and Karma tests with merged coverage report.
This enables the Vitest migration. From now on, we could use either

• pnpm test -f <package_name> to run tests for a single package with automatic Vitest/Karma detection, or
• vitest run --project <package_name> to run migrated packages.

Warning

Enablig Vitest requires changes in ckeditor5-dev to be merged and released. The new version should be bumped in dependencies: ckeditor/ckeditor5-dev#1317.

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💡 Additional information

Optional: Notes on decisions, edge cases, or anything helpful for reviewers.

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🧾 Checklists

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Author checklist

• Is the changelog entry intentionally omitted?
• Is the change backward-compatible?
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• Have tests been added that fail without this change (against regression)?
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Reviewer checklist

• PR description explains the changes and the chosen approach (especially when performance, API, or UX is affected).
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