Fix interleaved output in JsonReader's parseJsonNode method

Replace parallelStream with stream to prevent thread-unsafe appends to
the shared StringBuilder. This fixes the issue of intermingled key-value
pairs in the generated Document content. Also, replace StringBuffer
with StringBuilder for better performance in single-threaded context.

The change ensures correct ordering of extracted JSON keys and their
values in the resulting Document, improving the reliability and
readability of the parsed output.

spring-ai-core/src/main/java/org/springframework/ai/reader/JsonReader.java

@@ -42,19 +42,19 @@
  */
 public class JsonReader implements DocumentReader {
 
-	private Resource resource;
+	private final Resource resource;
 
-	private JsonMetadataGenerator jsonMetadataGenerator;
+	private final JsonMetadataGenerator jsonMetadataGenerator;
 
 	private final ObjectMapper objectMapper = new ObjectMapper();
 
 	/**
 	 * The key from the JSON that we will use as the text to parse into the Document text
 	 */
-	private List<String> jsonKeysToUse;
+	private final List<String> jsonKeysToUse;
 
 	public JsonReader(Resource resource) {
-		this(resource, new ArrayList<>().toArray(new String[0]));
+		this(resource, new String[0]);
 	}
 
 	public JsonReader(Resource resource, String... jsonKeysToUse) {
@@ -92,9 +92,9 @@ public List<Document> get() {
 	private Document parseJsonNode(JsonNode jsonNode, ObjectMapper objectMapper) {
 		Map<String, Object> item = objectMapper.convertValue(jsonNode, new TypeReference<Map<String, Object>>() {
 		});
-		StringBuffer sb = new StringBuffer();
+		var sb = new StringBuilder();
 
-		jsonKeysToUse.parallelStream().filter(item::containsKey).forEach(key -> {
+		jsonKeysToUse.stream().filter(item::containsKey).forEach(key -> {
 			sb.append(key).append(": ").append(item.get(key)).append(System.lineSeparator());
 		});