Renames one option of a custom tag category, everywhere its name is stored.

Body: { projectId, categoryName, oldValue, newValue }

Separate from /updateProjectCustomTagCategories on purpose: that endpoint replaces the project's categories wholesale, so a renamed option arrives as a deletion plus an insertion and there is no way to tell it apart from someone genuinely swapping one tag for another. A rename has to be stated to be carried out — see project_operations.rename_custom_tag_option for what it touches.