Fix four_forward_slashes false positive on inner doc comments

[zakrad]

four_forward_slashes scans the lines above an item for //// comments and suggests turning them into ///. It derives the item's start line from a span folded over the item's doc-comment attributes, but that fold included inner doc comments (//!).

An inner //! inside a body lowers onto the item with AttrStyle::Inner; folding its span pushes the item's end line down into the body, so the upward scan then flags a regular //// comment there. Applying the suggestion rewrites it to ///, which documents nothing and fails to compile (E0585).

Repro from the issue:

pub fn main() {
    //// I am not a doc comment!
    //! another comment
}

Fix: fold in outer doc comments only, mirroring the existing pattern in clippy_lints/src/doc/suspicious_doc_comments.rs. A genuine //// typo above an item still lints, and I added a regression test for the inner-doc case.

Fixes #16168

changelog: [four_forward_slashes]: no longer fires on //// comments that sit above an inner doc comment (//!) in an item's body

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Disclosure: I used AI assistance to help diagnose the root cause and draft this change. It's a small, self-contained fix that I've reviewed, tested locally (cargo uitest + cargo dev fmt), and can explain.

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[DanielEScherzer]

community review: looks good to me

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