Follow-up: docs and test-coverage hardening for native collect_list / array_agg (#4720)

[andygrove]

Follow-up to address review feedback from PR #4720 (native collect_list / array_agg): #4720 (comment)

The PR merges the core functionality; the items below are docs and test-coverage hardening deferred to a follow-on so they don't block the feature. All were flagged Medium severity.

1. Add a collect_set audit entry

docs/source/contributor-guide/expression-audits/agg_funcs.md documents ## collect_list but has no collect_set entry, even though PR #4720 modifies CometCollectSet semantically (new RESPECT NULLS branch via CometCollectShim.ignoreNulls(CollectSet), name-matched in hasNativeArrayBufferAgg). Add a ## collect_set section alongside ## collect_list (covering Spark 3.4.3 / 3.5.8 / 4.0.1 / 4.1.1, the SparkCollectSet delegation, the buffer-shape mismatch, and the NaN-dedup Incompatible reason on float/double), and replace the "same pattern already in use for collect_set" prose in the collect_list entry with a link to the new section.

2. Regression-cover the split-execution cascade

spark/src/test/scala/org/apache/comet/exec/CometAggregateSuite.scala — neither new collect_list test toggles COMET_ENABLE_FINAL_HASH_AGGREGATE / COMET_ENABLE_PARTIAL_HASH_AGGREGATE, so the hasNativeArrayBufferAgg + tagUnsafePartialAggregates path is untested. Mirror the existing "mixed engine sum/avg" tests (lines ~211/224) with:

• mixed engine collect_list: Comet partial + Spark final matches Spark
• mixed engine collect_list: Spark partial + Comet final matches Spark (the fallback case where Comet cannot read Spark's BinaryType buffer — adjustOutputForNativeState would misinterpret Binary state as an Array if it ran natively)

3. Assert the fallback reason in the distinct-combined test

In the new "collect_list/collect_set combined with distinct aggregate falls back safely" test, checkSparkAnswer doesn't verify anything ran/fell-back as expected. Use checkSparkAnswerAndFallbackReason (CometTestBase.scala:296) to assert the exact reason so the #4724 guard is actually protected:

Partial aggregate disabled: part of a multi-stage CollectList/CollectSet aggregate whose intermediate buffer cannot round-trip in Comet (issue #4724)

Apply to both the collect_list and collect_set variants, and to both the LocalTableScan and Parquet branches.

4. Extend SQL fixture datatype coverage

spark/src/test/resources/sql-tests/expressions/aggregate/collect_list.sql exercises timestamp but not timestamp_ntz (which is in QueryPlanSerde.supportedDataType, so it flows through the native SparkCollectList accumulator — an untested native path), and no year-month / day-time interval types (required by the "every datatype Spark accepts, NULLs in every column" rule; these exercise the Spark fallback). Append timestamp_ntz and ANSI interval blocks (both Spark 3.4+, already the file's floor). Gate the interval block with -- MinSparkVersion: 3.4 if it can't persist to Parquet on some target versions.