Fix panic when decoding multiple Union columns in RowConverter

[friendlymatthew]

Describe the bug

A panic occurs in the Union type row decoding logic where row slices aren't properly advanced when multiple columns are present

The decoder is prematurely consuming the entire row slice inside the field collection loop, causing subsequent columns to fail decoding (the actual panic)

[alamb]

label_issue.py automatically added labels {'arrow'} from #9000