Use transform_region() for prettify/unprettify

[rolandwalker]

Description

This method is built in to prompt_toolkit, and simpler. The position of the cursor in the transformed text is still not precise, as expected. (It wasn't precise before.)

xref #1646 covers the bot's main issue.

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: parse failures now wipe the entire query buffer

• In key_bindings.py and key_bindings.py, transform_region(...) is called unconditionally when buffer is non-empty.
• handle_prettify_binding() / handle_unprettify_binding() return "" on parse failure (main.py, main.py).
• Before this PR, empty result was explicitly ignored, preserving user text; now failed prettify/unprettify can replace the whole buffer with empty text (data loss regression).
• Action: keep old guard behavior by only applying transformed text when result is non-empty (or change handlers to return original input on parse failure).

2. Missing regression tests for failure/edge behavior

• Current coverage in test_main.py and test_main.py only checks successful formatting.
• Add tests for invalid SQL and multi-statement input verifying Ctrl-X p/u does not clear buffer and that toolbar error is set. This would have caught the regression above.

No new security issue found in this diff.