test: docs and test-coverage hardening for native collect_list / collect_set

[andygrove]

Which issue does this PR close?

Closes #5024.

Rationale for this change

PR #4720 landed the native collect_list / array_agg implementation and deferred four docs and test-coverage items to a follow-on so they would not block the feature. This PR addresses those items.

Two of them close real gaps in the regression net: neither collect_list test toggled the partial/final hash aggregate configs, so the hasNativeArrayBufferAgg + tagUnsafePartialAggregates split-execution path was untested; and the distinct-combined test only compared answers, so it would have passed even if the #4724 guard stopped firing and the query silently ran natively.

What changes are included in this PR?

1. collect_set audit entry. docs/source/contributor-guide/expression-audits/agg_funcs.md documented collect_list but not collect_set, even though feat: native collect_list / array_agg aggregate #4720 changed CometCollectSet semantically. Adds a ## collect_set section covering Spark 3.4.3 / 3.5.8 / 4.0.1 / 4.1.1, the SparkCollectSet delegation, the BinaryType-vs-list buffer-shape mismatch, and the NaN-dedup Incompatible reason on float/double. The collect_list entry now links to it instead of referring to it in prose.

2. Split-execution cascade regression tests. Two new tests in CometAggregateSuite, mirroring the existing mixed engine sum/avg pair:

   • mixed engine collect_list: Comet partial + Spark final matches Spark (COMET_ENABLE_FINAL_HASH_AGGREGATE=false)
   • mixed engine collect_list: Spark partial + Comet final matches Spark (COMET_ENABLE_PARTIAL_HASH_AGGREGATE=false), the case where adjustOutputForNativeState would misinterpret Spark's BinaryType buffer as a list if the final ran natively.

3. Fallback-reason assertions in the distinct-combined test. checkSparkAnswer is replaced with checkSparkAnswerAndFallbackReason(s). The two branches are protected by different guards, so each asserts its own:

   • LocalTableScan branch: the Partial was never convertible, so tagUnsafePartialAggregates does not fire and the dependent stages fall back via the buffer-source check (Comet aggregate that merges intermediate buffers requires a Comet child aggregate ... Incompatible aggregate function(s): collect_list).
   • Parquet branch: the Partial would otherwise convert, so the Support fully-native multi-stage (distinct-combined) collect_list / collect_set #4724 tagging reason (Partial aggregate disabled: part of a multi-stage CollectList/CollectSet aggregate whose intermediate buffer cannot round-trip in Comet (issue #4724)) is asserted alongside the buffer-source reason.

4. SQL fixture datatype coverage. collect_list.sql gains a timestamp_ntz block (in QueryPlanSerde.supportedDataType, so it runs through the native SparkCollectList accumulator, previously untested) and a year-month / day-time ANSI interval block (not supported natively, so spark_answer_only covers the Spark fallback). No MinSparkVersion gate is needed: both work on 3.4, which is already the file's floor.

How are these changes tested?

CometAggregateSuite and CometSqlFileTestSuite were run against both ends of the supported range, Spark 3.4 and the default Spark 4.1, and pass on both:

./mvnw test -Dsuites="org.apache.comet.exec.CometAggregateSuite collect_list" -Dtest=none
./mvnw test -Dsuites="org.apache.comet.CometSqlFileTestSuite collect_list" -Dtest=none

The NaN / -0.0 deduplication behavior asserted in the new audit entry was verified against Spark directly rather than inferred: collect_set over (0.0, -0.0, NaN, NaN, 1.0) returns [0.0, 1.0, NaN, NaN], confirming Spark keeps each NaN while collapsing +0.0 and -0.0.