Fix .NET context test: avoid SendAndWait with uncached prompt

Same issue as Node.js - the test harness proxy doesn't have
a cached CAPI response for 'Say hello'. Just create the session
and check listSessions without sending a message.

Author: jmoseley

dotnet/test/SessionTests.cs

@@ -373,9 +373,6 @@ public async Task SendAndWait_Blocks_Until_Session_Idle_And_Returns_Final_Assist
     public async Task Should_List_Sessions_With_Context()
     {
         var session = await Client.CreateSessionAsync();
-        await session.SendAndWaitAsync(new MessageOptions { Prompt = "Say hello" });
-
-        await Task.Delay(500);
 
         var sessions = await Client.ListSessionsAsync();
         Assert.NotEmpty(sessions);