Fix flaky .NET ask-user E2E tests (#2107)

Author: SteveSandersonMS

dotnet/test/E2E/AskUserE2ETests.cs

@@ -30,13 +30,11 @@ public async Task Should_Invoke_User_Input_Handler_When_Model_Uses_Ask_User_Tool
             }
         });
 
-        await session.SendAsync(new MessageOptions
+        await session.SendAndWaitAsync(new MessageOptions
         {
             Prompt = "Ask me to choose between 'Option A' and 'Option B' using the ask_user tool. Wait for my response before continuing."
         });
 
-        await TestHelper.GetFinalAssistantMessageAsync(session);
-
         // Should have received at least one user input request
         Assert.NotEmpty(userInputRequests);
 
@@ -62,13 +60,11 @@ public async Task Should_Receive_Choices_In_User_Input_Request()
             }
         });
 
-        await session.SendAsync(new MessageOptions
+        await session.SendAndWaitAsync(new MessageOptions
         {
             Prompt = "Use the ask_user tool to ask me to pick between exactly two options: 'Red' and 'Blue'. These should be provided as choices. Wait for my answer."
         });
 
-        await TestHelper.GetFinalAssistantMessageAsync(session);
-
         // Should have received a request
         Assert.NotEmpty(userInputRequests);