Soft-link or unlink a PIM product to an asset

Sets or clears pim_product_id on an asset without modifying equipment_model_id or equipment_make_id. Pass a UUID to soft-link the asset to that product. Pass null to clear the existing link. The field itself is required so callers must be explicit about intent — omitting it is rejected as a malformed request. When a UUID is supplied, the product must exist and belong to the asset's company or the global PIM tenant. The unlink path performs no PIM lookup. Bypasses the standard PIM make/model derivation and enrollment checks; intended for the read-only PIM matching workflow.