Sets or clears the selected product variant (e.g. color/finish/SKU) on a single line item — unrelated to the estimate-level CPQ good/better/best proposal_options (see Update Estimate Option). The line item must reference a catalog product (custom/manual line items are rejected), and the product must define option_values. Omitting option_uid clears the selection. Pass bundle_line_item_uid when line_item_uid is one of a BUNDLE's associated_products rather than a top-level line item. No pricing recalculation occurs — option values carry no price delta in this schema.