Fix orphaned OnDisposed callback when resuming sessions

When ResumeSessionAsync replaces an existing session, clear the old
session's OnDisposed callback to prevent it from firing SessionDestroyed
if disposed later.

Added test to verify old session disposal doesn't fire spurious events.

Author: anirsharma

dotnet/src/Client.cs

@@ -439,6 +439,13 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
             session.RegisterPermissionHandler(config.OnPermissionRequest);
         }
 
+        // Clear OnDisposed on the old session to prevent it from firing SessionDestroyed
+        // if it gets disposed after being replaced
+        if (_sessions.TryGetValue(response.SessionId, out var oldSession))
+        {
+            oldSession.OnDisposed = null;
+        }
+
         // Replace any existing session entry to ensure new config (like permission handler) is used
         _sessions[response.SessionId] = session;
 

dotnet/test/SessionTests.cs

@@ -212,6 +212,31 @@ public async Task Should_Fire_SessionCreated_When_Session_Is_Resumed()
         Assert.Same(session2, resumedSession);
     }
 
+    [Fact]
+    public async Task Should_Not_Fire_SessionDestroyed_When_Old_Session_Is_Disposed_After_Resume()
+    {
+        var session1 = await Client.CreateSessionAsync();
+        var sessionId = session1.SessionId;
+
+        var destroyedIds = new List<string>();
+        Client.SessionDestroyed += id => destroyedIds.Add(id);
+
+        // Resume creates a new session object for the same session ID
+        var session2 = await Client.ResumeSessionAsync(sessionId);
+
+        // Disposing the old session object should NOT fire SessionDestroyed
+        // because session2 is now the active session for this ID
+        await session1.DisposeAsync();
+
+        Assert.Empty(destroyedIds);
+
+        // Disposing the new session should fire SessionDestroyed
+        await session2.DisposeAsync();
+
+        Assert.Single(destroyedIds);
+        Assert.Equal(sessionId, destroyedIds[0]);
+    }
+
     [Fact]
     public async Task Should_Abort_A_Session()
     {