Add Debug.Assert that Completion is already completed before awaiting it

ConnectAsync's DisposeAsync already awaits Completion, so by the time
CreateAsync's catch block runs, it's guaranteed to be completed. The
assert documents this invariant.

Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/69e63301-5b59-4d6e-bf84-d1e02e421c67

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/McpClient.Methods.cs

@@ -1,6 +1,7 @@
 using Microsoft.Extensions.Logging;
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
+using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Text.Json;
 using System.Text.Json.Nodes;
@@ -57,6 +58,7 @@ public static async Task<McpClient> CreateAsync(
             // ConnectAsync already disposed the session (which includes awaiting Completion).
             // Check if the transport provided structured completion details indicating
             // why the transport closed that aren't already in the original exception chain.
+            Debug.Assert(clientSession.Completion.IsCompleted, "Completion should already be finished after ConnectAsync's DisposeAsync.");
             var completionDetails = await clientSession.Completion.ConfigureAwait(false);
 
             // If the transport closed with a non-graceful error (e.g., server process exited)