chore(audit): audit BitAndAgg and expand tests

[andygrove]

Which issue does this PR close?

N/A. Autonomous audit pass.

Rationale for this change

Audit of the bit_and expression (BitAndAgg) against Spark 3.4.3, 3.5.8, and 4.0.1. The Spark source is identical across all three versions, and Comet delegates the aggregate to DataFusion's bit_and_udaf. Static review found no behavioural divergence and no missing getIncompatibleReasons/getUnsupportedReasons overrides. Existing SQL coverage only exercised int with a single nullable row; this audit expands coverage to the remaining integral types and additional edge cases.

What changes are included in this PR?

• Audit sub-bullets in spark_expressions_support.md for Spark 3.4.3, 3.5.8, 4.0.1
• Expanded expressions/aggregate/bit_agg.sql: all integral types (tinyint, smallint, int, bigint), all-NULL group, empty input, HAVING clause, DISTINCT, boundary values (Int.MinValue/MaxValue, Long.MinValue/MaxValue), and a literal argument

How are these changes tested?

• ./mvnw test -DwildcardSuites=CometSqlFileTestSuite -Dsuites="org.apache.comet.CometSqlFileTestSuite bit_agg" -Dtest=none -Dscalastyle.skip=true (passes locally)

Note: the audit also drafted a bit_and OVER (...) window-form test, but removed it because Comet's WindowExec is gated behind spark.comet.operator.WindowExec.allowIncompatible=true due to known correctness issues unrelated to bit_and. That fallback path is not specific to this expression.

Scaffolded by the audit-comet-expression-autonomous skill.