fix(fmt: skip): respect skip directive on multi-line compound statements

[armorbreak001]

Summary

Fixes #5112

When # fmt: skip is placed on the colon line of a multi-line class (or def/if) statement, Black still reformatted it — collapsing multi-line type parameters onto a single line.

Root cause: In _generate_ignored_nodes_from_fmt_skip(), line 647 unconditionally sets prev_sibling = parent.prev_sibling when the leaf (NEWLINE) has no previous sibling. This causes the function to take the per-sibling traversal path (which stops at any leaf containing a newline in its prefix) instead of the suite-sibling collection path (the elif branch at line 738, which correctly collects ALL previous siblings of the suite node).

For a multi-line class like:

class Foo(Base[\
    "A", "B"\
]):  # fmt: skip

The traversal starts at : and collects ], ), : — then hits ] whose prefix contains \n (end of line), stopping early. The entire class header (class Foo(Base[, type params) is never marked as ignored, so Black reformats it.

Fix: Add parent.type != syms.suite condition on line 646 so that suite NEWLINEs fall through to the elif branch that properly collects all statement header nodes.

Test plan

• Reproduced issue # fmt: skip regression #5112 with the exact example from the bug report — now left unchanged
• Verified existing # fmt: skip cases still work (assignments, one-line compounds, etc.)
• All 210 format tests pass
• All 19 fmtskip format tests pass
• All 3 skip-related unit tests pass

[pekkaklarck]

Thanks for taking a look at this and for the quick fix @armorbreak001! I don't know the rules of this project, but in general I believe it would be a good idea to add a new test that reproduces the bug and is fixed by this change.

[cobaltt7]

Thanks @armorbreak001! Yes, please do add a new test case, as well as a changelog entry.

[github-actions]

diff-shades results comparing this PR (dded6c6) to main (8dcc486):

--preview style: no changes

--stable style: no changes

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