Removing incorrect end comma in jsonStructure for POJO extracting AI …

…Services (#894)

## Context
I noted that for more complex POJO object, the JSON returned by the LLM
often contained a last comma before closing bracket, making the JSON
invalid and resulting in an error while parsing it. I traced it back to
the proposed jsonStructure that is part of the SystemMessage, sending a
wrong format to the LLM.

## Change
Removed the last comma that shouldn't be there in the jsonStructure +
updated the unit tests

---------

Co-authored-by: Lize

langchain4j/src/main/java/dev/langchain4j/service/ServiceOutputParser.java

@@ -130,6 +130,7 @@ private static String jsonStructure(Class<?> structured, Set<Class<?>> visited)
             }
             jsonSchema.append(format("\"%s\": (%s),\n", name, descriptionFor(field, visited)));
         }
+        jsonSchema.delete(jsonSchema.lastIndexOf(","), jsonSchema.lastIndexOf(",")+1);
         jsonSchema.append("}");
         return jsonSchema.toString();
     }

langchain4j/src/test/java/dev/langchain4j/service/ServiceOutputParserTest.java

@@ -25,7 +25,7 @@ void outputFormatInstructions_SimplePerson() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: date string (2023-12-31)),\n" +
+                        "\"birthDate\": (type: date string (2023-12-31))\n" +
                         "}");
     }
 
@@ -43,7 +43,7 @@ void outputFormatInstructions_PersonWithFirstNameList() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: array of string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: date string (2023-12-31)),\n" +
+                        "\"birthDate\": (type: date string (2023-12-31))\n" +
                         "}");
     }
 
@@ -61,7 +61,7 @@ void outputFormatInstructions_PersonWithFirstNameArray() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: array of string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: date string (2023-12-31)),\n" +
+                        "\"birthDate\": (type: date string (2023-12-31))\n" +
                         "}");
     }
 
@@ -79,7 +79,7 @@ void outputFormatInstructions_PersonWithJavaType() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: java.util.Calendar),\n" +
+                        "\"birthDate\": (type: java.util.Calendar)\n" +
                         "}");
     }
 
@@ -98,7 +98,7 @@ void outputFormatInstructions_PersonWithStaticFinalField() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: date string (2023-12-31)),\n" +
+                        "\"birthDate\": (type: date string (2023-12-31))\n" +
                         "}");
     }
 
@@ -127,8 +127,8 @@ void outputFormatInstructions_PersonWithNestedObject() {
                         "\"address\": (type: dev.langchain4j.service.ServiceOutputParserTest$Address: {\n" +
                         "\"streetNumber\": (type: integer),\n" +
                         "\"street\": (type: string),\n" +
-                        "\"city\": (type: string),\n" +
-                        "}),\n" +
+                        "\"city\": (type: string)\n" +
+                        "})\n" +
                         "}");
     }
 
@@ -151,8 +151,8 @@ void outputFormatInstructions_PersonWithNestedObjectList() {
                         "\"address\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$Address: {\n" +
                         "\"streetNumber\": (type: integer),\n" +
                         "\"street\": (type: string),\n" +
-                        "\"city\": (type: string),\n" +
-                        "}),\n" +
+                        "\"city\": (type: string)\n" +
+                        "})\n" +
                         "}");
     }
 
@@ -175,8 +175,8 @@ void outputFormatInstructions_PersonWithNestedObjectArray() {
                         "\"address\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$Address: {\n" +
                         "\"streetNumber\": (type: integer),\n" +
                         "\"street\": (type: string),\n" +
-                        "\"city\": (type: string),\n" +
-                        "}),\n" +
+                        "\"city\": (type: string)\n" +
+                        "})\n" +
                         "}");
     }
 
@@ -200,7 +200,7 @@ void outputFormatInstructions_PersonWithFinalFields() {
                 "\nYou must answer strictly in the following JSON format: {\n" +
                         "\"firstName\": (type: string),\n" +
                         "\"lastName\": (type: string),\n" +
-                        "\"birthDate\": (type: date string (2023-12-31)),\n" +
+                        "\"birthDate\": (type: date string (2023-12-31))\n" +
                         "}");
     }
 
@@ -221,8 +221,8 @@ void outputFormatInstructions_PersonWithParents() {
                         "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParents: {\n" +
                             "\"firstName\": (type: string),\n" +
                             "\"lastName\": (type: string),\n" +
-                            "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParents),\n" +
-                            "}),\n" +
+                            "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParents)\n" +
+                            "})\n" +
                         "}");
     }
 
@@ -243,8 +243,8 @@ void outputFormatInstructions_PersonWithParentArray() {
                         "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParentArray: {\n" +
                             "\"firstName\": (type: string),\n" +
                             "\"lastName\": (type: string),\n" +
-                            "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParentArray),\n" +
-                            "}),\n" +
+                            "\"parents\": (type: array of dev.langchain4j.service.ServiceOutputParserTest$PersonWithParentArray)\n" +
+                            "})\n" +
                         "}");
     }
 
@@ -267,9 +267,9 @@ void outputFormatInstructions_PersonWithMotherAndFather() {
                             "\"firstName\": (type: string),\n" +
                             "\"lastName\": (type: string),\n" +
                             "\"mother\": (type: dev.langchain4j.service.ServiceOutputParserTest$PersonWithMotherAndFather),\n" +
-                            "\"father\": (type: dev.langchain4j.service.ServiceOutputParserTest$PersonWithMotherAndFather),\n" +
+                            "\"father\": (type: dev.langchain4j.service.ServiceOutputParserTest$PersonWithMotherAndFather)\n" +
                         "}),\n" +
-                        "\"father\": (type: dev.langchain4j.service.ServiceOutputParserTest$PersonWithMotherAndFather),\n" +
+                        "\"father\": (type: dev.langchain4j.service.ServiceOutputParserTest$PersonWithMotherAndFather)\n" +
                         "}");
     }