Use prompt_toolkit's bell()

[rolandwalker]

Description

This works the same way, but since we use prompt_toolkit, already, we should take more advantage of what it has built in.

Checklist

• I added this contribution to the changelog.md file.
• I added my name to the AUTHORS file (or it's already there).
• To lint and format the code, I ran

  uv run ruff check && uv run ruff format && uv run mypy --install-types .

[github-actions]

Findings
No correctness or security issues found in the PR diff.

Test Gaps / Edge Cases

1. Missing regression test for the new bell path at mycli/main.py:1004.
   Add a test that exercises beep_after_seconds > 0 and verifies the prompt-toolkit bell call is used (or at least that the path executes without error).

Assumptions

• self.prompt_app is always initialized before this branch runs inside run_cli().

Change Summary

• Replaced custom bell implementation with prompt_toolkit output bell call.
• Removed dead MyCli.bell() helper and updated changelog.