[PolicyRecordingBot] calls not cleaned up

It seems it can happen we miss the event that a call has ended. When we then send the heartbeat keepalive message for such a call, the system will reply with 404 call not found. We should then cleanup resources in the application. The heartbeat handler in the sample does not do this.

        protected override Task HeartbeatAsync(ElapsedEventArgs args)
        {
            return this.Call.KeepAliveAsync();
        }

it is only logged and forgotten:

        private void HeartbeatDetected(object sender, ElapsedEventArgs args)
        {
            var task = $"{this.GetType().FullName}.{nameof(this.HeartbeatAsync)}(args)";
            var taskObj = this.HeartbeatAsync(args);
            if (taskObj == null) return;
            this.GraphLogger.Verbose($"Starting running task: " + task);
            _ = Task.Run(() => taskObj).ForgetAndLogExceptionAsync(this.GraphLogger, task);
        }

Start with the PolicyRecordingBot sample's HeartbeatAsync and HeartbeatDetected handlers, then trace how KeepAliveAsync failures are logged. Ensure a 404 call-not-found response leads to application resource cleanup rather than only being forgotten, and verify the heartbeat path still handles other failures as before.