Change _eventHandlers from HashSet to ConcurrentDictionary for thread safety

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

Author: Copilot

dotnet/src/Session.cs

@@ -4,6 +4,7 @@
 
 using Microsoft.Extensions.AI;
 using StreamJsonRpc;
+using System.Collections.Concurrent;
 using System.Text.Json;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
@@ -44,7 +45,7 @@ namespace GitHub.Copilot.SDK;
 /// </example>
 public partial class CopilotSession : IAsyncDisposable
 {
-    private readonly HashSet<SessionEventHandler> _eventHandlers = new();
+    private readonly ConcurrentDictionary<SessionEventHandler, bool> _eventHandlers = new();
     private readonly Dictionary<string, AIFunction> _toolHandlers = new();
     private readonly JsonRpc _rpc;
     private PermissionRequestHandler? _permissionHandler;
@@ -245,8 +246,8 @@ void Handler(SessionEvent evt)
     /// </example>
     public IDisposable On(SessionEventHandler handler)
     {
-        _eventHandlers.Add(handler);
-        return new OnDisposeCall(() => _eventHandlers.Remove(handler));
+        _eventHandlers.TryAdd(handler, true);
+        return new OnDisposeCall(() => _eventHandlers.TryRemove(handler, out _));
     }
 
     /// <summary>
@@ -258,7 +259,7 @@ public IDisposable On(SessionEventHandler handler)
     /// </remarks>
     internal void DispatchEvent(SessionEvent sessionEvent)
     {
-        foreach (var handler in _eventHandlers.ToArray())
+        foreach (var handler in _eventHandlers.Keys)
         {
             // We allow handler exceptions to propagate so they are not lost
             handler(sessionEvent);