Add AI request model support and improve type handling

Refactored collection state management to handle API type changes and AI request models. Updated widgets and tests to support nullable HTTP methods and AI request models, and improved response body rendering for AI responses.

test/models/request_models.dart

@@ -218,7 +218,8 @@ Map<String, dynamic> requestModelJson = {
   'message': null,
   'httpResponseModel': responseModelJson,
   'preRequestScript': null,
-  'postRequestScript': null
+  'postRequestScript': null,
+  'aiRequestModel': null
 };
 
 /// Basic GET request model for apidash.dev