[Repo Assist] Regression test for lambda with MultiLineLambdaClosingNewline and changelog entry (#2553)

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This supersedes #3228, adding both the regression test and the changelog entry requested by @nojaf.

Closes #2553

Summary

Adds a regression test confirming that a lambda expression inside a chained method call (e.g. .Setup(fun ... -> <@ ... @>)) is formatted correctly when MultiLineLambdaClosingNewline = true and MaxLineLength = 100. The closing ) should not be placed on its own line.

Also adds a changelog entry under ## [Unreleased] documenting the fix.

Correct output

Mock()
    .Setup(fun aaaaaaaaaaaa ->
        <@ aaaaaaaaaaaa.Abcdefghijklmnopqrs "Food" "IsTastier" @>
    )
    .Returns(false)
    .Create()

Test Status

✅ dotnet build src/Fantomas.Core.Tests/ succeeds. The new regression test passes.

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