Fix UI flicker issue by updating prompt before alert message.

While testing the new background alerter thread, I noticed a slight UI flicker issue could occur when both msg and prompt are provided in an alert:
   - prompt_toolkit's patch_stdout() intercepts the printed message and immediately forces a redraw of the prompt so the message displays cleanly above it.
   - In the branch's implementation, self.prompt was being updated after printing the message. This caused patch_stdout to redraw the old prompt, and then get_app().invalidate() would immediately redraw the new prompt, causing a flicker.

I've updated cmd2/cmd2.py so that self.prompt is updated before the message is printed. This allows patch_stdout to natively draw the new prompt immediately, eliminating the flicker:

Author: tleonhardt

cmd2/cmd2.py

@@ -3367,20 +3367,22 @@ def _process_alerts(self) -> None:
                 # Get the next alert while still holding the condition lock.
                 alert = self._alert_queue.get()
 
-            if alert.msg:
-                # Print the message above the current prompt.
-                with patch_stdout():
-                    print_formatted_text(pt_filter_style(alert.msg))
-
             # Only apply prompt changes generated after the current prompt started.
+            prompt_updated = False
             if (alert.prompt is not None and
                 alert.prompt != self.prompt and
                 alert.timestamp > self._alert_prompt_timestamp):  # fmt: skip
                 self.prompt = alert.prompt
+                prompt_updated = True
+
+            if alert.msg:
+                # Print the message above the current prompt.
+                with patch_stdout():
+                    print_formatted_text(pt_filter_style(alert.msg))
 
-                # Refresh UI immediately unless at a continuation prompt.
-                if not self._at_continuation_prompt:
-                    get_app().invalidate()
+            # Refresh UI immediately unless at a continuation prompt.
+            if prompt_updated and not self._at_continuation_prompt:
+                get_app().invalidate()
 
     def _read_command_line(self, prompt: str) -> str:
         """Read the next command line from the input stream.

examples/async_printing.py

@@ -129,7 +129,7 @@ def _generate_colored_prompt(self) -> str:
         """Randomly generates a colored prompt
         :return: the new prompt.
         """
-        rand_num = random.randint(1, 20)
+        rand_num = random.randint(1, 6)
 
         status_color = Color.DEFAULT