Fixed flaky test UnsignedLongFieldBlockLoaderTests (elastic#141559)

* Fixed flaky test UnsignedLongFieldBlockLoaderTests

* Addressed feedback

Author: Kubik42

test/framework/src/main/java/org/elasticsearch/index/mapper/NumberFieldBlockLoaderTestCase.java

@@ -11,6 +11,7 @@
 
 import org.elasticsearch.datageneration.FieldType;
 
+import java.math.BigDecimal;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -48,21 +49,52 @@ protected Object expected(Map<String, Object> fieldMapping, Object value, TestCo
         return maybeFoldList(resultList);
     }
 
-    @SuppressWarnings("unchecked")
     private T convert(Object value, T nullValue, Map<String, Object> fieldMapping) {
-        if (value == null) {
-            return nullValue;
-        }
-        // String coercion is true by default
-        if (value instanceof String s && s.isEmpty()) {
-            return nullValue;
-        }
-        if (value instanceof Number n) {
-            return convert(n, fieldMapping);
+        switch (value) {
+            case null -> {
+                return nullValue;
+            }
+
+            // String coercion is true by default
+            case String s -> {
+                if (s.isEmpty()) {
+                    return nullValue;
+                }
+                // Attempt to parse the string as a number. If that fails, the string is malformed, so return null
+                Number parsed = tryParseString(s);
+                if (parsed != null) {
+                    return convert(parsed, fieldMapping);
+                }
+                return null;
+            }
+
+            case Number n -> {
+                return convert(n, fieldMapping);
+            }
+
+            default -> {
+                // Malformed values are excluded
+                return null;
+            }
         }
+    }
 
-        // Malformed values are excluded
-        return null;
+    /**
+     * Tries to parse a string as a number, matching the behavior of UnsignedLongFieldMapper.parseUnsignedLong().
+     * Returns null if the string cannot be parsed as a valid number.
+     */
+    private Number tryParseString(String s) {
+        try {
+            return Long.parseUnsignedLong(s);
+        } catch (NumberFormatException ex1) {
+            try {
+                BigDecimal bd = new BigDecimal(s);
+                return bd.toBigIntegerExact();
+            } catch (NumberFormatException | ArithmeticException ex2) {
+                // not a valid number
+                return null;
+            }
+        }
     }
 
     protected abstract T convert(Number value, Map<String, Object> fieldMapping);