Stabilize pragma comment width handling (#28795)
Summary
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I wasn't really planning to do a full stabilization pass, but I quickly skimmed through `preview.rs`
and remembered that the isort pragma change was kind a bug fix but technically breaking and thought
we might want to stabilize it.
This PR stabilizes the isort pragma comment width handling introduced in #27313, along with the
related pragma handling for `E501` and `W505` from #24071. There was a comment about also
stabilizing the related formatter preview feature on the function from #24071, so that is stabilized
here too.
I'm fine holding off on this, especially if the formatter change is a better fit for the 2027 style
guide. Like I said above, I mostly wanted to stabilize the isort change, but that snowballed a bit
since the helpers were shared.
Test Plan
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Updated existing snapshots
