fix(dynamodb-enhanced): Handle unicode control characters in toJson function (#5055)

The JsonParseException was occurring due to the toJson function
of EnhancedDocument not handling unicode control characters properly.
This fix ensures that unicode control characters are properly escaped
using backslashes to be included in string values, preventing the
Illegal unquoted character exception

Author: joviegas

.changes/next-release/bugfix-AmazonDynamoDBEnhancedClient-b245395.json

@@ -0,0 +1,6 @@
+{
+    "type": "bugfix",
+    "category": "Amazon DynamoDB Enhanced Client",
+    "contributor": "",
+    "description": "Fixed toJson function in EnhancedDocument API to properly handle unicode control characters by escaping them with backslashes, preventing JsonParseException."
+}

services-custom/dynamodb-enhanced/src/main/java/software/amazon/awssdk/enhanced/dynamodb/internal/document/JsonStringFormatHelper.java

@@ -75,7 +75,11 @@ public static String addEscapeCharacters(String input) {
                     output.append("\\\""); // double-quote character
                     break;
                 default:
-                    output.append(ch);
+                    if (Character.isISOControl(ch)) {
+                        output.append(String.format("\\u%04X", (int) ch));
+                    } else {
+                        output.append(ch);
+                    }
                     break;
             }
         }

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/document/EnhancedDocumentTest.java

@@ -64,6 +64,7 @@ private static Stream<Arguments> escapeDocumentStrings() {
             , Arguments.of("\n", "{\"key\":\"\\n\"}")
             , Arguments.of("\r", "{\"key\":\"\\r\"}")
             , Arguments.of("\f", "{\"key\":\"\\f\"}")
+            , Arguments.of("\u001a \u001A", "{\"key\":\"\\u001A \\u001A\"}")
         );
     }
 

services-custom/dynamodb-enhanced/src/test/java/software/amazon/awssdk/enhanced/dynamodb/document/EnhancedDocumentTestData.java

@@ -104,6 +104,17 @@ private void initializeTestData() {
 
                                       .build());
 
+        testDataList.add(dataBuilder().scenario("unicodeString")
+                                      .ddbItemMap(map().withKeyValue("stringKey", AttributeValue.fromS("\u001a \u000e")).get())
+                                      .enhancedDocument(
+                                          ((DefaultEnhancedDocument.DefaultBuilder)
+                                              DefaultEnhancedDocument.builder()).putObject("stringKey", "\u001a \u000e")
+                                                                                .addAttributeConverterProvider(defaultProvider()).build())
+                                      .attributeConverterProvider(defaultProvider())
+                                      .json("{\"stringKey\":\"\\u001A \\u000E\"}")
+
+                                      .build());
+
         testDataList.add(dataBuilder().scenario("record")
 
                                       .ddbItemMap(map().withKeyValue("uniqueId", AttributeValue.fromS("id-value"))