[jayzhan211]

Which issue does this PR close?

• None. This PR only adds tests, so there is no bug or feature issue to close.

Rationale for this change

The planner skips sorts and keeps streaming execution when it can prove that the data already has the needed
order. It decides this from what each plan node knows about its output: which orderings hold, which columns are
constant, and which columns are equal. Several of these decisions have no test today:

• a union that stays sorted when each side has a cast of the sort key in a different column,
• a constant from a filter that survives a sort with a limit or a cross join,
• the order in which GROUP BY keys are matched against the input's orderings,
• an order-preserving repartition over an unbounded input that keeps a cast of its sort key sorted.

Two open PRs show why this matters. #26459 and #26042 change how constants pass through outer joins. Cross
join derives its output facts as if it were a full outer join, so both would make cross-join queries sort on
columns that are still constant, and no existing test fails. The cross-join cases added here fail with either PR
merged in.

Without tests, a change to how the planner tracks these facts could add sorts, change the aggregation mode, or
make a streaming query fail to plan, and nothing would catch it. This PR pins the plans that main produces today.

What changes are included in this PR?

A new datafusion/sqllogictest/test_files/equivalence_properties.slt, with one comment per case that says which
planner decision it pins:

• A union whose sides each have a cast of a in a different column is still known to be sorted by a.
• GROUP BY d, c, b, a over data sorted by (a, b) and by c finds c, a, b sorted and runs in partially
  sorted mode on them.
• A filter constant survives an ORDER BY ... LIMIT, so an outer sort that starts with that constant is removed.
• A cross join keeps the constants of its left input.
• A cross join keeps a right-side constant set by an equality filter.
• Over an unbounded input, an order-preserving repartition keeps a cast of its sort key sorted, so a window
  partitioned by that cast needs no sort.

In datafusion/core/tests/physical_optimizer/enforce_distribution.rs, a new test checks that sort-merge join
keys are realigned to a range partitioning when the second of two orderings of an unbounded input matches it, not
only the first. The existing ordered_stream helper now takes a list of orderings so the new test can reuse it.

What is the testing strategy for this PR?

This PR adds tests only. All new cases pass on the current main branch, and the new sqllogictest file gives the
same result over repeated runs.

Are there any user-facing changes?

No.

[jayzhan211]

Thanks @zhuqi-lucas !