Reorder row groups by GROUP BY keys to reduce aggregate partition state and improve cache locality

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

#21317 introduced reordering row groups by statistics for **sort pushdown** (ORDER BY / TopK queries). The same mechanism could be extended to **GROUP BY** queries to improve aggregate performance.

As @adriangb and @Dandandan suggested in https://github.com/apache/datafusion/pull/21580#discussion_r3071295010:

> Ordering by grouping keys can:
> - Reduce cardinality within partitions (partition state can be smaller)
> - Allow for better cache locality (row groups with more equal keys are grouped together)

## Describe the solution you'd like

The existing `PreparedAccessPlan::reorder_by_statistics` method already accepts any `LexOrdering` — it's generic, not tied to sort pushdown. Extending this for GROUP BY would be:

1. In the aggregate planner, compute a preferred row group ordering from the grouping keys
2. Pass it through `ParquetSource::sort_order_for_reorder`
3. Existing reorder logic handles the rest

```
SELECT category, SUM(x) FROM t GROUP BY category

Before: RGs in random order
→ HashAggregate sees mixed categories
→ Hash table stays large (all categories in memory)
→ Cache misses as different categories interleave

After: RGs ordered by category's min statistics
→ HashAggregate sees category-grouped rows
→ Can finalize/flush groups earlier (streaming aggregation opportunity)
→ Better cache locality (same category rows adjacent)
```

## Describe alternatives you've considered

- **Streaming aggregation** requires fully sorted input, which is a stronger guarantee. RG-level reorder doesn't guarantee full sort but improves locality within each RG.

## Additional context

- Parent: #21317 (row group reorder for sort pushdown)
- Related PR: #21580 (adds the `reorder_by_statistics` infrastructure)

The infrastructure from #21580 should be directly reusable — this issue is mainly about wiring up the GROUP BY planner to populate `sort_order_for_reorder` based on grouping keys.

Start in the aggregate planner and trace how grouping keys could populate ParquetSource::sort_order_for_reorder. Read PreparedAccessPlan::reorder_by_statistics and the infrastructure from #21580, then inspect #21317 for the existing sort-pushdown path. Done means GROUP BY plans request row-group ordering by their grouping keys and the relevant aggregate behavior is covered by tests.

Written by the indexing model from the issue text.