Preserve blank lines before fmt-off classes

[sjh9714]

Description

Fixes #5225.

When a top-level class starts inside a # fmt: off block after an import, the
converted block looks like a standalone comment to the empty-line tracker. This caused
Black to keep only one blank line instead of the two required before a top-level class.

Preserve the converted block's original class token when deciding import separation.
The change is limited to a new preview feature, so stable formatting remains unchanged,
and a focused fixture covers the reported input.

Testing

• .venv/bin/pytest tests/test_format.py -k preview_fmt_off_class_blank_lines -q
• .venv/bin/pytest tests/test_format.py -q
• env -u NO_COLOR .venv/bin/pytest -q
• env -u NO_COLOR .venv/bin/pre-commit run mypy --files src/black/lines.py src/black/mode.py
• env -u NO_COLOR .venv/bin/tox -e run_self
• .venv/bin/tox -e generate_schema

env -u NO_COLOR .venv/bin/pre-commit run --all-files passed every hook except
repository-wide mypy, which reports an unchanged action/main.py tomllib
ignore/stub mismatch under Python 3.14. The targeted mypy hook above passes for both
changed Python modules.

Note: I used Codex while preparing this change, reviewed the final diff, and ran the
checks listed above locally.

Checklist - did you ...

• Implement any code style changes under the --preview style, following the stability policy?
• Add an entry in CHANGES.md if necessary?
• Add / update tests if necessary?
• Add new / update outdated documentation?

[github-actions]

diff-shades results comparing this PR (7371f49) to main (51abf53):

--preview style: no changes

--stable style: no changes

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