Which issue does this PR close?

• Closes Intermittent failures in CI in test_files/limit.slt #16180.

Rationale for this change

Intermittent test failures were observed in the limit.slt SQL logic test file on the main branch, specifically related to a query that uses a LIMIT clause without an ORDER BY. Since SQL LIMIT behavior is non-deterministic without a defined order, this led to flaky test results.

This change explicitly adds an ORDER BY part_key to the LIMIT subquery to ensure deterministic results and resolve test flakiness.

What changes are included in this PR?

• Modified the SQL logic test to include ORDER BY part_key before the LIMIT clause.
• Updated the expected logical and physical plans accordingly:
  • The logical plan now includes a Sort operator before the limit.
  • The physical plan includes a SortPreservingMergeExec to maintain global ordering across partitions.
  • Replaces the limited DataSourceExec with a combination of SortExec and TopK on each partition to correctly apply the limit post-sort.

Are these changes tested?

Yes. The change modifies an existing SQL logic test to enforce a deterministic result, which now consistently passes in CI runs.

Tested with:

for i in {1..100}; do
  echo "Run #$i"
  cargo test -p datafusion-sqllogictest limit || break
done

Are there any user-facing changes?

No. This change only affects internal test stability and does not impact the user-facing API or behavior.