Push down topk through join

[SubhamSinghal]

Which issue does this PR close?

#11900

Rationale for this change

When a query has ORDER BY <cols> LIMIT N on top of an outer join and all sort columns come from the preserved side,
DataFusion currently runs the full join first, then sorts and limits. We can push a copy of the Sort(fetch=N) to the preserved input, reducing the number of rows entering the join.

Before:

Sort: t1.b ASC, fetch=3
   Left Join: t1.a = t2.a
     Scan: t1     ← scans ALL rows
     Scan: t2

After:

  Sort: t1.b ASC, fetch=3
    Left Join: t1.a = t2.a
      Sort: t1.b ASC, fetch=3  ← pushed down
        Scan: t1               ← only top-3 rows enter join
      Scan: t2

What changes are included in this PR?

A new logical optimizer rule PushDownTopKThroughJoin that:

1. Matches Sort with fetch = Some(N) (TopK)
2. Looks through an optional Projection to find a Join
3. Checks join type is LEFT or RIGHT with no non-equijoin filter
4. Verifies all sort expression columns come from the preserved side
5. Inserts a copy of the Sort(fetch=N) on the preserved child
6. Keeps the top-level sort for correctness

Are these changes tested?

Yes through UT

Are there any user-facing changes?

No API changes.