Add SDK MCP OAuth host token handlers

dotnet/src/Client.cs

@@ -572,6 +572,7 @@ private CopilotSession InitializeSession(
             this);
         session.RegisterTools(config.Tools ?? []);
         session.RegisterPermissionHandler(config.OnPermissionRequest);
+        session.RegisterMcpAuthHandler(config.OnMcpAuthRequest);
         session.RegisterCommands(config.Commands);
         session.RegisterElicitationHandler(config.OnElicitationRequest);
         session.RegisterExitPlanModeHandler(config.OnExitPlanModeRequest);
@@ -878,6 +879,10 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
                 transformCallbacks,
                 hasHooks,
                 "CopilotClient.CreateSessionAsync");
+            if (config.OnMcpAuthRequest is not null)
+            {
+                await session.Rpc.EventLog.RegisterInterestAsync("mcp.oauth_required", cancellationToken);
+            }
         }
         try
         {
@@ -987,6 +992,12 @@ public async Task<CopilotSession> CreateSessionAsync(SessionConfig config, Cance
                     $"session.create returned sessionId {response.SessionId} but the caller requested {localSessionId}.");
             }
 
+            // Local IDs registered before create; server-assigned IDs can only register now.
+            if (localSessionId is null && config.OnMcpAuthRequest is not null)
+            {
+                await session.Rpc.EventLog.RegisterInterestAsync("mcp.oauth_required", cancellationToken);
+            }
+
             session.WorkspacePath = response.WorkspacePath;
             session.SetCapabilities(response.Capabilities);
             session.SetOpenCanvases(response.OpenCanvases);
@@ -1073,6 +1084,10 @@ public async Task<CopilotSession> ResumeSessionAsync(string sessionId, ResumeSes
             transformCallbacks,
             hasHooks,
             "CopilotClient.ResumeSessionAsync");
+        if (config.OnMcpAuthRequest is not null)
+        {
+            await session.Rpc.EventLog.RegisterInterestAsync("mcp.oauth_required", cancellationToken);
+        }
 
         try
         {