[Kristofferjoha]

Which issue does this PR close?

Closes #26745.

Rationale for this change

An order-preserving repartition can incorrectly retain independent input orderings that do not hold after merging partitions. This lets the planner remove a necessary sort, causing ORDER BY to return rows in the wrong order.

What changes are included in this PR?

• Retain only the merge key ordering when repartitioning merges multiple input partitions.
• Preserve projected cast relationships so valid cast-derived orderings can still be inferred after the merge.
• Extend ordering checks to use equivalent expressions without assuming that equality implies a strictly order-preserving transformation.
• Update affected expected plans.

What is the testing strategy for this PR?

• Add the issue’s reproduction to order.slt, verifying exact row order.
• Add safety cases in equivalence_properties.slt and monotonic_projection_test.slt.
• Verify the existing unbounded-stream cast-ordering test continues to pass.

Are there any user-facing changes?

Only that queries affected by #26745 now return rows in the requested order.

[Kristofferjoha]

@jayzhan211 This should be ready for review. It needed a little more than following #26654, as keeping only the merge key ordering also removed valid cast-derived orderings, causing an existing unbounded-stream query to fail planning. This PR also preserves projected cast relationships so those valid orderings can be inferred again.