Fix ObjectDisposedException when disposing session after client.StopAsync()

Make CopilotSession.DisposeAsync() idempotent and tolerant to already-disposed
connections by adding an _isDisposed flag and catching ObjectDisposedException
and IOException during disposal.

Fixes #306

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

dotnet/src/Session.cs

@@ -54,6 +54,7 @@ public partial class CopilotSession : IAsyncDisposable
     private SessionHooks? _hooks;
     private readonly SemaphoreSlim _hooksLock = new(1, 1);
     private SessionRpc? _sessionRpc;
+    private int _isDisposed;
 
     /// <summary>
     /// Gets the unique identifier for this session.
@@ -560,8 +561,24 @@ await InvokeRpcAsync<object>(
     /// </example>
     public async ValueTask DisposeAsync()
     {
-        await InvokeRpcAsync<object>(
-            "session.destroy", [new SessionDestroyRequest() { SessionId = SessionId }], CancellationToken.None);
+        if (Interlocked.Exchange(ref _isDisposed, 1) == 1)
+        {
+            return;
+        }
+
+        try
+        {
+            await InvokeRpcAsync<object>(
+                "session.destroy", [new SessionDestroyRequest() { SessionId = SessionId }], CancellationToken.None);
+        }
+        catch (ObjectDisposedException)
+        {
+            // Connection was already disposed (e.g., client.StopAsync() was called first)
+        }
+        catch (IOException)
+        {
+            // Connection is broken or closed
+        }
 
         _eventHandlers.Clear();
         _toolHandlers.Clear();

dotnet/test/ClientTests.cs

@@ -215,4 +215,13 @@ public void Should_Throw_When_UseLoggedInUser_Used_With_CliUrl()
             });
         });
     }
+
+    [Fact]
+    public async Task Should_Not_Throw_When_Disposing_Session_After_Stopping_Client()
+    {
+        await using var client = new CopilotClient(new CopilotClientOptions());
+        await using var session = await client.CreateSessionAsync();
+
+        await client.StopAsync();
+    }
 }