Preserve multiline headers with fmt skip

[17X61]

Fixes #5112.

Summary

• Handle # fmt: skip on compound statement colon lines before same-line backtracking.
• Preserve multiline class/function headers as a whole when the skip comment is on the colon line.
• Add regression coverage for multiline class, function, async function, and parenthesized if headers.

Test Plan

• python3 -m pytest tests/test_format.py -k fmtskip_class_header
• Previously ran the full formatting suite on this branch before the extra edge-case fixture additions.

[Copilot]

Pull request overview

Fixes a regression in Black’s # fmt: skip handling where multiline class/function headers could be collapsed when the skip comment appears on the compound statement’s colon line (e.g. class ...): # fmt: skip).

Changes:

• Prioritize detection of # fmt: skip on compound-statement colon lines before the generic “same physical line” backtracking logic to preserve multiline headers intact.
• Keep special handling for async statements when collecting ignored nodes for colon-line skips.
• Add a regression fixture covering a multiline class base list with # fmt: skip on the colon line.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
tests/data/cases/fmtskip_class_header.py	Adds a regression case ensuring multiline class headers remain unchanged when # fmt: skip is on the colon line.
src/black/comments.py	Adjusts ignored-node generation ordering so colon-line # fmt: skip preserves entire multiline compound headers.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[cobaltt7]

@17X61 Please expand the tests to include more edge cases

[github-actions]

diff-shades results comparing this PR (41bd05c) to main (9fd9ea2):

--preview style: no changes

--stable style: no changes

---

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

[17X61]

Added more regression coverage for multiline colon-line # fmt: skip cases: function headers, async function headers, and parenthesized if headers, in addition to the original class header case.

Validation run locally:

python3 -m pytest tests/test_format.py -k fmtskip_class_header