Add CollectLeft partition mode to SortMergeJoinExec

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

SortMergeJoinExec currently always runs in a symmetric, hash-partitioned mode:
both inputs are hash-partitioned on the join keys and sorted, and partition i
of the left is merge-joined with partition i of the right.

When one side is small, hash-repartitioning the large side is wasteful. For
hash joins this is already handled by PartitionMode::CollectLeft (collect the
small build side once, broadcast it, don't repartition the probe side). This PR
brings the same idea to sort-merge joins.

### Describe the solution you'd like

Implement CollectLeft mode for SMJ

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by tracing the existing PartitionMode::CollectLeft implementation for hash joins and the SortMergeJoinExec execution path. Verify how the small left side is collected and broadcast, then compare sort-merge join behavior with symmetric partitioning; done means CollectLeft avoids repartitioning the large side while producing correct join results.

Written by the indexing model from the issue text.