Fix case guard splitting at small line lengths

[puneetdixit200]

Fixes #4280

Description

When case case if True is formatted at a small line length, Black can make the pattern parentheses visible but also wrap the guard itself. That produces invalid output shaped like case case ( if True ):, and the second formatting pass fails to parse it.

This keeps guard nodes from being wrapped as a whole and allows the case-pattern split to keep the visible pattern parentheses instead. The guard remains outside the pattern parentheses, matching the expected output from the issue.

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• Add new / update outdated documentation?

Tests run:

• .\.venv\Scripts\python -m pytest tests/test_format.py -k pattern_matching_case_case_small_line_length -q
• .\.venv\Scripts\python -m pytest tests/test_format.py -k pattern_matching -q
• .\.venv\Scripts\python -m pytest tests/test_format.py -q
• .\.venv\Scripts\python -m black --check src\black\linegen.py
• .\.venv\Scripts\python -m pytest tests -q --run-optional no_jupyter -k "not symlink"

I also ran .\.venv\Scripts\python -m pytest tests -q --run-optional no_jupyter; only three existing symlink tests failed because this Windows environment cannot create symlinks.

[github-actions]

diff-shades results comparing this PR (0da5688) to main (7613840):

--preview style: no changes

--stable style: no changes

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What is this? | Workflow run | diff-shades documentation

[cobaltt7]

Thanks!