[Analytics Engine] Map BigDecimal cells to FloatingPoint in row-codec inference

{@code RowResponseCodec.scalarArrowType} ordered its instanceof checks
{Long, Integer, Short, Byte, Double, Float, Boolean, CharSequence, byte[],
Number(fallback) → Int(64)}. BigDecimal extends {@link Number} but isn't any
of the typed scalar arms, so it fell through to the {@code Number} fallback
and got encoded as a 64-bit integer column — silently truncating fractional
digits.

This bites PPL flows whose common element type is {@code DECIMAL} (e.g.
{@code array(1, -1.5, 2, 1.0)} — the v2-side {@code ArrayImplementor.internalCast}
explicitly maps the DECIMAL target to BigDecimal cells). The element values
{@code -1.5} and {@code 1.0} round to {@code -1} and {@code 1} when forced
through Int(64), so the array reads back as {@code [1, -1, 2, 1]} instead of
{@code [1, -1.5, 2, 1.0]}.

Promote BigDecimal cells to FloatingPoint(DOUBLE) — same precision the v2
engine uses for decimal-typed PPL results, so behavior matches across both
execution paths. The list writer's {@code Float8Vector} arm already uses
{@code ((Number) element).doubleValue()}, which correctly extracts the
fractional value from a BigDecimal.

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

Author: ahkcs

sandbox/plugins/analytics-engine/src/main/java/org/opensearch/analytics/exec/stage/RowResponseCodec.java

@@ -142,6 +142,13 @@ private static ArrowType scalarArrowType(Object value) {
         if (value instanceof Byte) return new ArrowType.Int(8, true);
         if (value instanceof Double) return new ArrowType.FloatingPoint(org.apache.arrow.vector.types.FloatingPointPrecision.DOUBLE);
         if (value instanceof Float) return new ArrowType.FloatingPoint(org.apache.arrow.vector.types.FloatingPointPrecision.SINGLE);
+        // BigDecimal must be checked before the Number fallback below — a BigDecimal that
+        // would round to a long but actually carries fractional precision (e.g. PPL
+        // {@code array(1, -1.5)} where the common element type is DECIMAL and
+        // {@code ArrayImplementor.internalCast} produces BigDecimal cells) would otherwise
+        // get encoded as an integer Arrow vector and lose its fractional digits. Promote to
+        // DOUBLE — the same path the v2 engine takes for decimal-typed PPL results.
+        if (value instanceof java.math.BigDecimal) return new ArrowType.FloatingPoint(org.apache.arrow.vector.types.FloatingPointPrecision.DOUBLE);
         if (value instanceof Boolean) return ArrowType.Bool.INSTANCE;
         if (value instanceof CharSequence) return ArrowType.Utf8.INSTANCE;
         if (value instanceof byte[]) return ArrowType.Binary.INSTANCE;