`add_sort_above` and `add_sort_above_with_distribution` pruned every
expression that equivalence properties reported as constant, via
`is_expr_constant(..).is_none()`. That treats `AcrossPartitions::Heterogeneous`
the same as `AcrossPartitions::Uniform`, which is wrong.

A `Heterogeneous` constant is constant *within* each partition but varies
*across* partitions (e.g. `sample` in
`SELECT 1 AS sample, c FROM t UNION ALL SELECT 2 AS sample, c FROM t`).
Both helpers build a partition-preserving `SortExec` whose output is merged
across partitions downstream, where such a column is still the leading
ordering discriminator. Dropping it silently discards the global ordering,
producing non-deterministic output order for `ORDER BY sample, c` once the
plan is rooted at a sink / output requirement.

Only `AcrossPartitions::Uniform` is globally constant and safe to drop.
Introduce `is_uniform_constant` and use it at both prune sites.

Adds a regression test that builds a union with a heterogeneous-constant
column and asserts the key survives `add_sort_above`.