fix(text_region): prevent font state leaking out of text_columns rendering

[SAY-5]

Fixes #1804

The render path inside _render_styled_text_line mutates self.font_size_pt to match each fragment but never restores it, so text_columns() leaves the FPDF instance with the font size of the last rendered fragment. Snapshot current_font, font_size_pt, and font_style at the start of TextColumns.render() and restore them in a finally block.

Checklist:

• A unit test is covering the code added / modified by this PR

• In case of a new feature, docstrings have been added, with also some documentation in the `docs/` folder

• A mention of the change is present in `CHANGELOG.md`

• This PR is ready to be merged

By submitting this pull request, I confirm that my contribution is made under the terms of the GNU LGPL 3.0 license.

[SAY-5]

Reworked this: the snapshot/restore now lives in ParagraphCollectorMixin.__exit__ around render() rather than inside TextColumns.render(), so HTML rendering (which calls render() directly without the context manager) keeps its font-state persistence and all of test_html.py stays green. Full test/text_region/ and test/html/ suites pass locally, and the regression test fails on master without the fix.