[AURON #1840] Preserve collect_set first-occurrence order

[peter941221]

What changed

AccSet::merge no longer swaps accumulators by set size. That swap changed the encounter order of collect_set, so no-shuffle Spark checks could see values in rhs-first order instead of first-occurrence order.

Why

Spark's collect_set preserves first-occurrence order in the no-shuffle path used by the affected aggregate suite.

Testing

• git diff --check
• cargo +nightly test --manifest-path native-engine/datafusion-ext-plans/Cargo.toml test_acc_set_merge -- --nocapture (blocked here by rdkafka-sys Windows build failure: %1 is not a valid Win32 application. (os error 193))

[peter941221]

Rebased this onto current master and kept the fix minimal. The change still just removes the size-based swap in AccSet::merge and adds order assertions for the merge cases. New CI is running on f092709.

[richox]

do we really need to keep this order? if i dont understand wrong, this order we be discarded in bucket-merge spill strategy even if we maintain it in updating

[peter941221]

@richox thanks for calling out the spill path. I checked that path and added a focused regression for it.

The spill bucket merge still comes back through the same collect_set merge path this PR changes:
AggTable::output bucket merge in agg_table.rs
-> partial_merge
-> merge_items in collect.rs
-> AccSet::merge in collect.rs

I also checked the spill round-trip itself. AccSetColumn::save_raw writes list.raw as-is, and unspill rebuilds it in order through load_raw / append_item, so spill does not discard the internal encounter order by itself.

To make that concrete, I pushed 645b3236, which adds test_acc_set_merge_preserves_first_occurrence_order_after_rhs_spill. It spills the larger RHS accumulator, unspills it, merges it into the LHS, and still gets [1, 2, 3] on the repo's pinned nightly-2025-05-09.

So I think we still need this fix for the spill path too.