Add process exit handler to TelegramBot

Introduce a new event handler for the `ProcessExit` event in the
`TelegramBot` namespace within `BotApp.cs`. The `OnProcessExit`
method logs an informational message and cancels the
`_cancellationTokenSource` when the process exit event is received.
This ensures proper cleanup and logging when the application exits.

Author: bvdcode

Sources/TelegramBot/BotApp.cs

@@ -99,6 +99,7 @@ public void Run(CancellationToken cancellationToken = default)
         public async Task StartAsync(CancellationToken cancellationToken = default)
         {
             var mergedToken = MergeTokens(cancellationToken);
+            AppDomain.CurrentDomain.ProcessExit += new EventHandler(OnProcessExit);
             CheckDisposed();
             try
             {
@@ -122,6 +123,12 @@ public async Task StartAsync(CancellationToken cancellationToken = default)
             }
         }
 
+        private void OnProcessExit(object sender, EventArgs e)
+        {
+            _logger.LogInformation("Process exit event received.");
+            _cancellationTokenSource.Cancel();
+        }
+
         /// <summary>
         /// Attempts to gracefully stop the program.
         /// </summary>