making mappedMessages immutable again

cognitive/src/main/scala/com/microsoft/azure/synapse/ml/services/openai/OpenAIChatCompletion.scala

@@ -132,21 +132,23 @@ class OpenAIChatCompletion(override val uid: String) extends OpenAIServicesBase(
   override def responseDataType: DataType = ChatCompletionResponse.schema
 
   private[this] def getStringEntity(messages: Seq[Row], optionalParams: Map[String, Any]): StringEntity = {
-    var mappedMessages: Seq[Map[String, String]] = messages.map { m =>
-      Seq("role", "content", "name").map(n =>
-        n -> Option(m.getAs[String](n))
-      ).toMap.filter(_._2.isDefined).mapValues(_.get)
+    // Convert each message row to a map of non-null values
+    val mappedMessages: Seq[Map[String, String]] = messages.map { messageRow =>
+      Seq("role", "content", "name").map { fieldName =>
+        fieldName -> Option(messageRow.getAs[String](fieldName))
+      }.toMap.filter(_._2.isDefined).mapValues(_.get)
     }
 
-    // if the optionalParams contains "response_format" key, and it's value contains "json_object",
-    // then we need to add a message to instruct openAI to return the response in JSON format
-    if (optionalParams.get("response_format")
-                      .exists(_.asInstanceOf[Map[String, String]]("type")
-                      .contentEquals("json_object"))) {
-      mappedMessages :+= Map("role" -> "system", "content" -> OpenAIResponseFormat.JSON.prompt)
+    // Check if the response format is JSON and add a system message if needed
+    val updatedMessages = if (optionalParams.get("response_format")
+      .exists(_.asInstanceOf[Map[String, String]]("type") == "json_object")) {
+      mappedMessages :+ Map("role" -> "system", "content" -> OpenAIResponseFormat.JSON.prompt)
+    } else {
+      mappedMessages
     }
 
-    val fullPayload = optionalParams.updated("messages", mappedMessages)
+    // Update the optional parameters with the messages
+    val fullPayload = optionalParams.updated("messages", updatedMessages)
     new StringEntity(fullPayload.toJson.compactPrint, ContentType.APPLICATION_JSON)
   }