Enumerate _eventHandlers directly instead of .Keys to avoid lock and allocation

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

Author: Copilot

dotnet/src/Session.cs

@@ -259,7 +259,7 @@ public IDisposable On(SessionEventHandler handler)
     /// </remarks>
     internal void DispatchEvent(SessionEvent sessionEvent)
     {
-        foreach (var handler in _eventHandlers.Keys)
+        foreach (var (handler, _) in _eventHandlers)
         {
             // We allow handler exceptions to propagate so they are not lost
             handler(sessionEvent);