[C++] Unsigned dictionary index types should use the full unsigned range #50695

Follow-up to #50475, suggested in review there: an unsigned dictionary index type keeps its signedness, but the width still adapts on the signed thresholds because the dictionary builder reuses the signed AdaptiveIntBuilder internally. A uint8 index widens to uint16 after 128 distinct values instead of 256, so one bit of the range is unused.