fix: JSON builder string encoding of control characters

src/main/java/org/hisp/dhis/jsontree/JsonAppender.java

@@ -104,15 +104,26 @@ void appendEscaped( CharSequence str ) {
             return;
         }
         append( '"' );
-        str.chars().forEachOrdered( c -> {
-            if ( c == '"' || c == '\\' || c < ' ' ) {
-                appendChar.accept( '\\' );
-            }
-            appendChar.accept( (char) c );
-        } );
+        str.chars().forEachOrdered( this::appendEscaped );
         append( '"' );
     }
 
+    private void appendEscaped( int c ) {
+        switch ( c ) {
+            case '\b' -> append( "\\b" );
+            case '\f' -> append( "\\f" );
+            case '\n' -> append( "\\n" );
+            case '\r' -> append( "\\r" );
+            case '\t' -> append( "\\t" );
+            case '"' -> append( "\\\"" );
+            case '\\' -> append( "\\\\" );
+            case -31 -> append( "\\u%04X".formatted( c ) );
+            case 0x2028 -> append( "\\u2028" );
+            case 0x2029 -> append( "\\u2029" );
+            default -> appendChar.accept( (char) c );
+        }
+    }
+
     private void beginLevel( char c ) {
         append( c );
         hasChildrenAtLevel[++level] = false;

src/main/java/org/hisp/dhis/jsontree/JuonAppender.java

@@ -213,12 +213,23 @@ private void append(char c) {
 
     void appendEscaped(@Surly CharSequence str ) {
         append( '\'' );
-        str.chars().forEachOrdered( c -> {
-            if ( c == '\'' || c == '\\' || c < ' ' ) {
-                append( '\\' );
-            }
-            append( (char) c );
-        } );
+        str.chars().forEachOrdered( this::appendEscaped );
         append( '\'' );
     }
+
+    private void appendEscaped( int c ) {
+        switch ( c ) {
+            case '\b' -> append(  "\\b" );
+            case '\f' -> append( "\\f" );
+            case '\n' -> append( "\\n" );
+            case '\r' -> append( "\\r" );
+            case '\t' -> append( "\\t" );
+            case '\'' -> append( "\\'" );
+            case '\\' -> append( "\\\\" );
+            case -31 -> append( "\\u%04X".formatted( c ) );
+            case 0x2028 -> append( "\\u2028" );
+            case 0x2029 -> append( "\\u2029" );
+            default -> append( (char) c );
+        }
+    }
 }

src/test/java/org/hisp/dhis/jsontree/JsonAppenderTest.java

@@ -269,6 +269,25 @@ void testArray_JsonNode() {
             .addElement( JsonNode.of( "[\"a\",\"b\"]" ) ) ) );
     }
 
+    @Test
+    void testNewlines() {
+        //language=JSON
+        String json = """
+            {"id":"woOg1dUFoX0","time":1738685143915,"message":"com.fasterxml.jackson.core.JsonParseException: Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')\\n at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 1, column: 1]","level":"ERROR"}""";
+        JsonObject msg = JsonMixed.of( json );
+        String message = msg.getString( "message" ).string();
+        assertEquals( """
+                com.fasterxml.jackson.core.JsonParseException: Unexpected character ('<' (code 60)): expected a valid value (JSON String, Number, Array, Object or token 'null', 'true' or 'false')
+                 at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 1, column: 1]""",
+            message );
+        JsonObject actual = Json.object( obj ->
+            obj.addString( "id", "woOg1dUFoX0" )
+                .addNumber( "time", 1738685143915L )
+                .addString( "message", message )
+                .addString( "level", "ERROR" ));
+        assertEquals( json, actual.toJson() );
+    }
+
     private static void assertJson( String expected, JsonNode actual ) {
         assertEquals( expected.replace( '\'', '"' ), actual.getDeclaration() );
     }

src/test/java/org/hisp/dhis/jsontree/JsonTest.java

@@ -35,8 +35,9 @@ void testOf_Boolean() {
     void testOf_String() {
         assertJson( "null", Json.of( (String) null ) );
         assertJson( "\"hello\"", Json.of( "hello" ) );
-        assertJson( "\"hello\\\n"
-            + "world\"", Json.of( "hello\nworld" ) );
+        JsonString hello = Json.of( "hello\nworld" );
+        assertJson( "\"hello\\nworld\"", hello );
+        assertEquals( "hello\nworld", hello.string() );
     }
 
     @Test