chore(audit): audit any_value and expand tests

[andygrove]

Which issue does this PR close?

N/A. Autonomous audit pass.

Rationale for this change

Audit of the any_value expression against Spark 3.4.3, 3.5.8, and 4.0.1.

AnyValue is a RuntimeReplaceableAggregate whose replacement is First(child, ignoreNulls), and the optimizer rule ReplaceExpressions rewrites it before physical planning. Comet therefore handles any_value transparently through CometFirst; no dedicated serde exists. The class is identical across the three Spark versions checked. Coverage in first_last.sql is comprehensive for first, but no test in the repo exercises the any_value name directly. A regression in the analyzer rewrite, in a future AnyValue.replacement, or in Comet's plan conversion would silently bypass that coverage.

What changes are included in this PR?

• Audit sub-bullets in spark_expressions_support.md for any_value across the three Spark versions.
• New SQL test file spark/src/test/resources/sql-tests/expressions/aggregate/any_value.sql exercising any_value directly: basic, with grouping, isIgnoreNull literal forms, all-null groups, mixing with other aggregates, and equivalence to first on identical inputs.

How are these changes tested?

./mvnw test -Dsuites="org.apache.comet.CometSqlFileTestSuite any_value" -Dtest=none passes locally (Spark 3.5 profile, 1 test, 0 failures).

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