Emit APPROX_COUNT_DISTINCT as the distinct_count_approx runtime name

PPLBuiltinOperators.DISTINCT_COUNT_APPROX created its SqlAggFunction with the
runtime-resolution name "DISTINCT_COUNT_APPROX". The analytics-engine (DataFusion)
backend resolves aggregates by the Calcite/Substrait-standard name
APPROX_COUNT_DISTINCT, so distinct_count_approx() failed to bind on the analytics
route. Emit APPROX_COUNT_DISTINCT instead.

The Java field name stays DISTINCT_COUNT_APPROX (the PPL function name); only the
resolution string changes. The OpenSearch V3 path is unaffected — it overrides this
operator via the external HyperLogLog registration in OpenSearchExecutionEngine
(whose name is unchanged), so explain output and execution on that path are
identical (verified). The analytics-route binding is completed by
opensearch-project/OpenSearch#22013 (APPROX_COUNT_DISTINCT -> approx_distinct).

Per Sandesh Kumar.

Signed-off-by: Kai Huang <ahkcs@amazon.com>

Author: ahkcs

core/src/main/java/org/opensearch/sql/expression/function/PPLBuiltinOperators.java

@@ -521,7 +521,11 @@ public class PPLBuiltinOperators extends ReflectiveSqlOperatorTable {
   public static final SqlAggFunction DISTINCT_COUNT_APPROX =
       createUserDefinedAggFunction(
           DistinctCountApproxLogicalAggFunction.class,
-          "DISTINCT_COUNT_APPROX",
+          // Emit the Calcite/Substrait-standard name so the analytics-engine (DataFusion) backend
+          // resolves it; the OpenSearch V3 path overrides this via the external HLL registration in
+          // OpenSearchExecutionEngine. The Java field name stays DISTINCT_COUNT_APPROX (the PPL
+          // function name); only this runtime-resolution string changes.
+          "APPROX_COUNT_DISTINCT",
           ReturnTypes.BIGINT_FORCE_NULLABLE,
           null);