fix(bot): prevent duplicate persistent error notifications on command check failures

[espdesign]

Description

Fixes an issue where users without required command permissions received two simultaneous error notifications—one temporary auto-dismissing toast and one persistent followup notification that remained on screen indefinitely.

Closes #46

Summary of Changes

1. Fallback Guard in DggPmBot.on_tree_error (bot.py):
   • Check command._has_any_error_handlers() to suppress duplicate dispatch if a cog or command-level error handler (cog_app_command_error) already handled the error.
   • Avoid blanket suppression on deferred interactions so unhandled errors occurring after deferral are still dispatched.
2. Followup Auto-Dismissal Targeting (error_handler.py):
   • Pass wait=True to interaction.followup.send(..., wait=True) so discord.py returns the WebhookMessage instead of None.
   • Pass the returned message to menu_manager.schedule_toast_dismissal, allowing ephemeral followup errors to be cleanly auto-dismissed.
3. Automated Regression Tests (test_error_handling.py):
   • Added end-to-end command check failure pipeline test verifying single response dispatch.
   • Added test ensuring followup messages are awaited with wait=True and scheduled for dismissal.
   • Verified that unhandled errors on deferred interactions still send followup error toasts.

Verification

• Ran full test suite via make check (397 tests passing, ruff lint and format check clean).