Use prompt_toolkit's clipboard for the \clip command

[rolandwalker]

Description

This currently has the same functionality as depending on pyperclip directly; however it simplifies things to depend only on prompt_toolkit, reducing the chance that there is ever a conflict over the pyperclip version declarations.

Incidentally remove a needless f-string.

Part of a series: trying to depend on prompt_toolkit more, handroll less, and depend on click less for output.

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]

1. High: packaging/runtime regression risk (pyperclip may no longer be installed).
   pyproject.toml:10 removes the explicit pyperclip dependency, but iocommands.py:14 and iocommands.py:249 still rely on prompt_toolkit’s pyperclip-backed clipboard class. If prompt_toolkit doesn’t guarantee installing pyperclip transitively, \clip can fail (or import-time behavior can differ by environment).
   Action: either keep pyperclip as a direct dependency, or switch to a prompt_toolkit clipboard implementation that does not require the external pyperclip package.

2. Medium: missing regression tests for this behavior change.
   No tests were added/updated for \clip path despite changing clipboard implementation and dependency behavior.
   Action: add tests in test/test_special_iocommands.py for:

• successful copy path (mock clipboard backend),
• backend failure path (assert user-facing error message),
• behavior when clipboard backend dependency is unavailable.

Could not run tests locally because pytest is not installed in this environment.

[rolandwalker]

Closing, because prompt_toolkit does not actually install pyperclip transitively. So there is no benefit.