MINOR: Fix validation of decimals at their declared precision (#1342)

DecimalUtilityl#checkPrecisionAndScaleNoThrow used to reject value
precision that was equal to the vector precision, it should not.

Author: robert3005

vector/src/main/java/org/apache/arrow/vector/util/DecimalUtility.java

@@ -118,7 +118,7 @@ public static boolean checkPrecisionAndScale(
    */
   public static boolean checkPrecisionAndScaleNoThrow(
       BigDecimal value, int vectorPrecision, int vectorScale) {
-    return value.scale() == vectorScale && value.precision() < vectorPrecision;
+    return value.scale() == vectorScale && value.precision() <= vectorPrecision;
   }
 
   /**

vector/src/test/java/org/apache/arrow/vector/TestDecimalPrecisionBoundary.java

@@ -0,0 +1,53 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.arrow.vector;
+
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.math.BigDecimal;
+import org.apache.arrow.memory.BufferAllocator;
+import org.apache.arrow.memory.RootAllocator;
+import org.apache.arrow.vector.util.DecimalUtility;
+import org.apache.arrow.vector.validate.ValidateUtil.ValidateException;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+
+public class TestDecimalPrecisionBoundary {
+  @ParameterizedTest
+  @ValueSource(ints = {128, 256})
+  public void testValidateScalarsAtDeclaredPrecision(int bitWidth) {
+    try (BufferAllocator allocator = new RootAllocator();
+        BaseFixedWidthVector vector =
+            bitWidth == 128
+                ? new DecimalVector("decimal", allocator, 5, 2)
+                : new Decimal256Vector("decimal", allocator, 5, 2)) {
+      vector.allocateNew(2);
+      DecimalUtility.writeBigDecimalToArrowBuf(
+          new BigDecimal("999.99"), vector.getDataBuffer(), 0, bitWidth / 8);
+      DecimalUtility.writeBigDecimalToArrowBuf(
+          new BigDecimal("-999.99"), vector.getDataBuffer(), 1, bitWidth / 8);
+      vector.setIndexDefined(0);
+      vector.setIndexDefined(1);
+      vector.setValueCount(2);
+      vector.validateScalars();
+      // Values beyond the declared precision must still be rejected.
+      DecimalUtility.writeBigDecimalToArrowBuf(
+          new BigDecimal("1000.00"), vector.getDataBuffer(), 0, bitWidth / 8);
+      assertThrows(ValidateException.class, vector::validateScalars);
+    }
+  }
+}