[Repo Assist] Parenthesise lambda LHS of infix operator in single-line form to preserve semantics (#3274)

[github-actions]

Closes #3274

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Problem

When a lambda appears as the left-hand side of a custom infix operator, and the entire expression fits on one line, Fantomas was emitting it without parentheses — changing the semantics of the code:

Input (4 variations, all with (fun x -> ...) <*| op semantics):

let a =
    fun x -> {| X = x |}
    <*| op

Before (wrong — changes meaning):

let a = fun x -> {| X = x |} <*| op

This parses as fun x -> ({| X = x |} <*| op) — the operator is now inside the lambda body.

After (correct):

let a = (fun x -> {| X = x |}) <*| op

Root Cause

In Expr.InfixApp handling, genShortInfixExpr (the single-line layout) calls genExpr node.LeftHandSide without adding parentheses. When the LHS is a lambda, fun x -> body op rhs is re-parsed as fun x -> (body op rhs), which is semantically different.

For newLineInfixOps (|>, >>, etc.), Fantomas already forces multiline layout when the LHS is a lambda. But for other operators (e.g. <*|, <$>, >>=), the short single-line form was used without guarding the semantics.

Fix

Add parentheses around the LHS expression in genShortInfixExpr when it is a lambda or if/then/else:

// When the LHS is a lambda or if/then/else, it must be parenthesised in the
// single-line form. Without parens, `fun x -> body (op) rhs` would be parsed
// as `fun x -> (body (op) rhs)` rather than `(fun x -> body) (op) rhs`, which
// changes semantics. See #3274.
let genLhsExpr =
    if isLambdaOrIfThenElse node.LeftHandSide then
        sepOpenT +> genExpr node.LeftHandSide +> sepCloseT
    else
        genExpr node.LeftHandSide

If the lambda was already explicitly parenthesised in the source, its Oak representation is Expr.Paren(Expr.Lambda(...)), which does not match isLambdaOrIfThenElse, so no double parens are added.

Test Status

• ✅ Build: dotnet build fantomas.sln — succeeded
• ✅ Tests: dotnet test src/Fantomas.Core.Tests/ — 2737 passed, 0 failed, 7 skipped
• ✅ New regression test added: lambda on LHS of custom infix operator is parenthesised to preserve semantics, 3274

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[nojaf]

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