fix(frontend): Sync request models up with forge.agent_protocol (#7202)

Frontend broke in #7171 because of changes to the request models in `forge.agent_protocol`. This PR unbreaks it.

Changes:
- Make `input` required on `TaskRequestBody` and `StepRequestBody`
- Amend `toJson()` on `TaskRequestBody` and `StepRequestBody` to omit attributes with `null` value

frontend/lib/models/step_request_body.dart

@@ -1,13 +1,12 @@
 class StepRequestBody {
-  final String? input;
+  final String input;
   final Map<String, dynamic>? additionalInput;
 
   StepRequestBody({required this.input, this.additionalInput});
 
   Map<String, dynamic> toJson() {
-    if (input == null && additionalInput == null) {
-      return {};
-    }
-    return {'input': input, 'additional_input': additionalInput};
+    Map<String, dynamic> result = {'input': input, 'additional_input': additionalInput};
+    result.removeWhere((_, v) => v == null);
+    return result;
   }
 }

frontend/lib/models/task_request_body.dart

@@ -5,6 +5,8 @@ class TaskRequestBody {
   TaskRequestBody({required this.input, this.additionalInput});
 
   Map<String, dynamic> toJson() {
-    return {'input': input, 'additional_input': additionalInput};
+    Map<String, dynamic> result = {'input': input, 'additional_input': additionalInput};
+    result.removeWhere((_, v) => v == null);
+    return result;
   }
 }

frontend/lib/viewmodels/chat_viewmodel.dart

@@ -111,7 +111,7 @@ class ChatViewModel with ChangeNotifier {
   }
 
   /// Sends a chat message for a specific task.
-  void sendChatMessage(String? message,
+  void sendChatMessage(String message,
       {required int continuousModeSteps, int currentStep = 1}) async {
     if (_currentTaskId == null) {
       print("Error: Task ID is not set.");
@@ -165,7 +165,7 @@ class ChatViewModel with ChangeNotifier {
       if (_isContinuousMode && !executedStep.isLast) {
         print("Continuous Mode: Step $currentStep of $continuousModeSteps");
         if (currentStep < continuousModeSteps) {
-          sendChatMessage(null,
+          sendChatMessage("",
               continuousModeSteps: continuousModeSteps,
               currentStep: currentStep + 1);
         } else {

frontend/lib/views/chat/chat_view.dart

@@ -117,7 +117,7 @@ class _ChatViewState extends State<ChatView> {
                 try {
                   if (widget.viewModel.currentTaskId != null) {
                     widget.viewModel.sendChatMessage(
-                        (message == "") ? null : message,
+                        message,
                         continuousModeSteps: Provider.of<SettingsViewModel>(
                                 context,
                                 listen: false)
@@ -126,7 +126,7 @@ class _ChatViewState extends State<ChatView> {
                     String newTaskId = await taskViewModel.createTask(message);
                     widget.viewModel.setCurrentTaskId(newTaskId);
                     widget.viewModel.sendChatMessage(
-                        (message == "") ? null : message,
+                        message,
                         continuousModeSteps: Provider.of<SettingsViewModel>(
                                 context,
                                 listen: false)