Improved import row grouping for files without IDs

Import files without clear identifiers like product IDs or handles now group rows more intelligently, using the “Top Row” column if present.

🔧 Improvements

• Top Row grouping – files without ID/handle columns now use “Top Row” markers to group related rows together.
• Fallback grouping – when no grouping column exists, all rows combine into a single object (matching Matrixify behavior).