JsonFieldPatternBuilder mask nested json field break json format

[AKKID]

Issue Report

When using the JsonFieldPatternBuilder to mask sensitive data within a nested JSON structure, the resulting output is not in a valid JSON format.

Sample code

public class EJMaskTest {
    static {
        EJMaskInitializer.addFilter(
                new BaseFilter(JsonFieldPatternBuilder.class, 4, "innerKey1")
        );
    }


    public static void main(String[] args) throws JsonProcessingException {
        String input = "{\"outer\":\"  {\\\"innerKey1\\\": \\\"191287127167167\\\",\\\"innerKey2\\\":\\\"testInnerInformation\\\"}\"}";
        String output = EJMask.mask(input);
        System.out.println(input);
        System.out.println(output);
    }
}

Below is the output

{"outer":"  {\"innerKey1\": \"191287127167167\",\"innerKey2\":\"testInnerInformation\"}"}
{"outer":"  {\"innerKey1\": \"1912-xxxx",\"innerKey2\":\"testInnerInformation\"}"}

Expected Behavior

The expected output should mask only the innerKey1 value from 191287127167167 to 1912-xxxx, while leaving all other content unchanged. The correct output should be:

{"outer":"  {\"innerKey1\": \"1912-xxxx\",\"innerKey2\":\"testInnerInformation\"}"}

Actual Behavior

The actual output is:

{"outer":"  {\"innerKey1\": \"1912-xxxx",\"innerKey2\":\"testInnerInformation\"}"}

There is a missing escape character (backslash) following 1912-xxxx, which results in an invalid JSON format.

Steps to Reproduce the Issue

1. Use the code provided in the "Sample Code" section.
2. Observe the output.