[Repo Assist] Stay multiline when list element is a tuple with lambda as last field (#3278)

[github-actions]

Closes #3278

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Problem

When a list is collapsed to a single-line form (elements separated by ;), if any list element is a tuple whose last element is a lambda or if/then/else, the lambda body captures the ; separator and subsequent list elements as a sequence expression — silently changing the semantics of the code.

Example:

// Input (2-element list):
let x =
    [
        1, fun () -> 1
        1, fun () -> 1
    ]

// Before (WRONG — 1-element list!):
let x = [ 1, fun () -> 1; 1, fun () -> 1 ]
// F# parses as: [ (1, fun () -> (1; 1, fun () -> 1)) ] — ONE element

Verified:

let x_fmt = [ 1, fun () -> 1; 1, fun () -> 1 ]
List.length x_fmt  // = 1, not 2 — semantics changed!

Root Cause

In genArrayOrList, the alwaysMultiline guard forces multiline layout when any element is if/then/else, or all elements are lambdas/ite. But 1, fun () -> 1 is an Expr.Tuple, not a bare lambda, so neither guard fires. The list passes the "fits on one line?" check and collapses — producing semantically incorrect code.

Fix

Extend the alwaysMultiline check with a wouldSwallowNextListItem helper: if any non-last element is a tuple whose last expression is a lambda or if/then/else, force multiline. In the multiline form, elements are on separate lines where whitespace-sensitivity scopes the lambda body correctly.

This follows the "stay multiline" design principle (Option B in RFC #3279), which is the approach the community overwhelmingly voted for (16:1 as of the time of this PR).

The fix is minimal and surgical — it only affects lists where a non-last element would cause a semantic change when collapsed.

Test Status

• ✅ Build: dotnet build fantomas.sln — succeeded
• ✅ Tests: dotnet test src/Fantomas.Core.Tests/ — 2738 passed, 0 failed, 7 skipped
• ✅ New regression test added: lambda in non-last tuple position in list stays multiline to preserve semantics, 3278

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[nojaf]

Gonna solve this in a larger refactor to have a more overall consistent story.