feat: support listagg / string_agg aggregate (Spark 4.0+)

[andygrove]

Which issue does this PR close?

Closes #.

Rationale for this change

Spark 4.0 added the LISTAGG / STRING_AGG aggregate to concatenate string values in a group. Comet previously fell back to Spark for the entire aggregate. This PR adds native support for the common shape so groups that use listagg on a StringType column with a literal delimiter can stay in Comet's native execution path.

What changes are included in this PR?

• Adds a Comet-owned SparkListAgg UDAF (native/spark-expr/src/agg_funcs/list_agg.rs) that skips nulls, returns Utf8, and — critically — keeps its intermediate state as Binary to match Spark's TypedImperativeAggregate buffer schema. Emitting Utf8 state would force the Comet shuffle layer to insert a Utf8 → Binary cast that the merge side then cannot read back.
• Adds the CometListAgg serde in spark/src/main/spark-4.x/ and wires it into QueryPlanSerde.aggrSerdeMap through a new sparkVersionSpecificAggregates shim slot (empty for Spark 3.x, provides ListAgg on Spark 4.x). This keeps the base map compiling against Spark 3.x where ListAgg does not exist.
• Adds a new ListAgg protobuf message and wires it through the native planner.
• Restricts native execution to the simple form. WITHIN GROUP (ORDER BY ...), BinaryType inputs, non-literal delimiters, and non-default collations fall back to Spark. DISTINCT falls back naturally because Comet's aggregate dispatcher rejects multi-column distinct aggregates.
• Documents the audit findings in docs/source/contributor-guide/expression-audits/agg_funcs.md and updates the support status in docs/source/user-guide/latest/expressions.md.

Scaffolding produced by the implement-comet-expression skill; the audit-comet-expression skill drove the fallback and coverage-gap tests.

How are these changes tested?

• New Rust unit tests for ListAggAccumulator covering delimiter handling, null-input skipping, empty groups, and multi-partition state merge (native/spark-expr/src/agg_funcs/list_agg.rs).
• New Comet SQL test at spark/src/test/resources/sql-tests/expressions/aggregate/listagg.sql gated to Spark 4.0+ (MinSparkVersion: 4.0), running under ConfigMatrix: parquet.enable.dictionary=false,true to exercise both dictionary-encoded and plain-encoded reads. It covers:
  • Native cases: literal delimiter, string_agg alias, default (empty) delimiter, all-NULL group, empty table, global aggregate, multi-char and empty-string delimiters, empty-string values inside a group, multibyte UTF-8 values (composed accents, CJK, emoji).
  • expect_fallback cases: DISTINCT, WITHIN GROUP (ORDER BY ...) ascending and descending, BinaryType child with default and literal binary delimiter.
• ./mvnw test -Pspark-4.0 -Dsuites="org.apache.comet.CometSqlFileTestSuite listagg" -Dtest=none passes for both dictionary-encoding modes.
• cd native && cargo clippy --all-targets --workspace -- -D warnings passes.