[CORE-372] Fix Highlight Clear Not-Found Handling

[dytsou]

Type of changes

• Fix
• Test

Purpose

• Return ErrHighlightNotFound when clearing a highlight that does not exist
• Add unit tests for highlight.Service.Clear with mock dependencies
• Resolve issue CORE-372

Additional Information

• Jira: https://clustron.atlassian.net/browse/CORE-372

[Dada8825252]

Suggest delegating the existence check to the delete operation itself — consider changing DeleteByFormID to :execrows so the service can inspect rowsAffected directly and remove the GetByFormID pre-check entirely. This simplifies the flow from two queries down to one, and also avoids the case where a concurrent delete causes the pre-check to pass but the delete to silently no-op.

[dytsou]

Suggest delegating the existence check to the delete operation itself — consider changing DeleteByFormID to :execrows so the service can inspect rowsAffected directly and remove the GetByFormID pre-check entirely.

Addressed: DeleteByFormID is now :execrows and Clear checks rowsAffected == 0 for ErrHighlightNotFound, removing the extra GetByFormID round-trip.