keep async keyword with fmt: skip one-line compound statements

[sahvx655-wq]

Description

Reported in #5307. Black crashes on a one-line async with/async for/async def that has a semicolon-separated body and a trailing # fmt: skip, e.g. async with ctx(): a; b # fmt: skip. The non-async form is fine, and the async form is fine without the semicolon, so it took a while to pin down the combination. The parse error it dies on is misleading (cannot parse: ... async) because it comes from Black's second pass reparsing its own output rather than from the input.

Dumping the first-pass output showed the cause: async had been split onto its own line, leaving async\n with g(): ..., which is not valid Python. When a one-line compound statement carries a semicolon body under # fmt: skip, _get_compound_statement_header gathers the header leaves to keep them inline, but it only walks the children of the with_stmt/for_stmt/funcdef. For an async statement the tree wraps that node in an async_stmt and the leading ASYNC token is a sibling of the compound node, not a child, so it never made it into the skipped nodes and got reformatted on its own. The colon-line form of # fmt: skip already handles this via the async_stmt grandparent, but the inline-semicolon path did not. The risk if left as-is is that any file with such a line cannot be formatted at all under --safe, and under --fast Black would silently emit broken code. Fixed by picking up the async_stmt's leading ASYNC token first; the three async variants are added to fmtskip10.py.

Close #5307, closes #5308, closes #5314

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• Add new / update outdated documentation?

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diff-shades results comparing this PR (85f309d) to main (74371e2):

--preview style: no changes

--stable style: no changes

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