Spring-ai Fix JsonReader parallel issue

This PR fixes an issue with parallel stream used in JsonReader.

Problem

I wrote a simple program to test the JsonReader. The JSON content is very simple.

[
  {
    "name": "Alex",
    "email": "alex@example.com",
    "jobTitle": "Software Engineer"
  },
  {
    "name": "Bob",
    "email": "bob@example.com",
    "jobTitle": "System Admin"
  }
]

The code shown below extracts keys name and jobTitle and puts them into the Document.

public class JsonReaderSample {

  void read() {
    var metadataGenerator = new JsonMetadataGenerator() {
      @Override
      public Map<String, Object> generate(Map<String, Object> jsonMap) {
        return Map.of("email", jsonMap.getOrDefault("email", ""));
      }
    };
    var resource = new FileSystemResource(
        Path.of(".", "data", "json-array.json"));
    var reader = new JsonReader(resource, metadataGenerator, "name",
        "jobTitle");
    var docs = reader.read();
    docs.forEach(System.out::println);
  }

  public static void main(String[] args) {
    new JsonReaderSample().read();
  }
}

When running this simple program, the output may look like below. The text of keys and values from different keys are mingled (jobTitle: name: System AdminBob).

Document{id='a55153c1-09ab-4fc6-aa18-07a6f20e94d6', metadata={email=alex@example.com}, content='name: Alex
jobTitle: Software Engineer
', media=[]}
Document{id='46b27f32-1ea3-4ca9-a005-9c93195bb335', metadata={email=bob@example.com}, content='jobTitle: name: System AdminBob

', media=[]}

The usage of parallelStream caused the invocations of StringBuffer.append for different keys from different threads intertwined when operating on the shared StringBuffer.

Fix

The parallel processing of multiple keys seems unnecessary, since parallel processing is already enabled for documents in the JSON array, and we are dealing with only in-memory data. So I changed to use normal stream and replaced StringBuffer with StringBuilder.

Comment From: markpollack

Yikes! thanks a ton.

merged in c205c7d5cad5d6278be9c56d3babe6ed9af87ae9