skip expensive ptr chase & comparission in ByteGroupValueBuilder

### Is your feature request related to a problem or challenge?

in `datafusion/datafusion/physical-plan/src/aggregates/group_values/multi_group_by/bytes.rs` at https://github.com/apache/datafusion/blob/7538aefbe9ee224814570798cd381d1948029efa/datafusion/physical-plan/src/aggregates/group_values/multi_group_by/bytes.rs#L173 we can avoid the expensive double buffer look up by doing a length comparison

### Describe the solution you'd like

short circuit buffer check if lengths mismatch

### Describe alternatives you've considered

n/a

### Additional context

_No response_

Start in datafusion/datafusion/physical-plan/src/aggregates/group_values/multi_group_by/bytes.rs at the ByteGroupValueBuilder code around line 173. Trace the current buffer comparison and confirm where lengths can be compared first; done means mismatched lengths avoid the buffer check without changing grouping behavior.

Written by the indexing model from the issue text.