fix(validate): Use BigDecimal for multipleOf to avoid float equality

SonarQube flagged the previous (n / multipleOf) % 1 != 0 check for
floating-point equality. Switches to BigDecimal.remainder so e.g. 0.3
divided by 0.1 reports cleanly instead of falling foul of double rounding.

Author: thced

src/main/java/com/retailsvc/http/validate/DefaultValidator.java

@@ -18,6 +18,7 @@
 import com.retailsvc.http.spec.schema.Schema;
 import com.retailsvc.http.spec.schema.StringSchema;
 import com.retailsvc.http.spec.schema.TypeName;
+import java.math.BigDecimal;
 import java.time.LocalDate;
 import java.time.OffsetDateTime;
 import java.util.ArrayList;
@@ -175,11 +176,22 @@ private void validateNumber(Object value, NumberSchema s, String pointer) {
     if (s.exclusiveMaximum() != null && n >= s.exclusiveMaximum().doubleValue()) {
       fail(pointer, "exclusiveMaximum", "number not less than " + s.exclusiveMaximum(), n);
     }
-    if (s.multipleOf() != null && (n / s.multipleOf().doubleValue()) % 1 != 0) {
+    if (s.multipleOf() != null && !isMultipleOf(n, s.multipleOf().doubleValue())) {
       fail(pointer, "multipleOf", "not a multiple of " + s.multipleOf(), n);
     }
   }
 
+  /**
+   * Returns whether {@code value} is an exact multiple of {@code divisor}, using {@link BigDecimal}
+   * to avoid floating-point rounding artifacts that {@code (value / divisor) % 1 == 0} would
+   * produce (e.g., {@code 0.3 / 0.1} is not exactly {@code 3.0} as a double).
+   */
+  private static boolean isMultipleOf(double value, double divisor) {
+    BigDecimal v = BigDecimal.valueOf(value);
+    BigDecimal d = BigDecimal.valueOf(divisor);
+    return v.remainder(d).compareTo(BigDecimal.ZERO) == 0;
+  }
+
   @SuppressWarnings("unchecked")
   private void validateObject(Object value, ObjectSchema s, String pointer) {
     require(value instanceof Map, pointer, "type", "expected object");