[Repo Assist] Fix soundness bug: %% (double-percent) infix operator moves to new line, breaking code

[github-actions]

🤖 This is an automated PR from Repo Assist.

Closes #2107

Problem

When a %% infix expression was too long to fit on one line, Fantomas placed the %% operator at the start of the next line:

let a =
    (if aVeryLongVariableNameThatForceLineBreaking = 0 then
         1
     else
         -1)
    %% 4   // ← invalid: F# parser reads this as prefix splice operator %%4
```

This produced **invalid F#**. The F# compiler interprets `%%` at the start of a line as the quotation splice operator `(~%%)`, expecting a `Quotations.Expr` argument — not an `int`. The error is:
```
error FS0001: This expression was expected to have type 'Quotations.Expr' but here has type 'int'

Root Cause

CodePrinter.fs maintains a noBreakInfixOps set of operators that must stay on the same line as their LHS (because starting a new line with them changes their parse meaning). The set correctly contained %, =, <, > — but was missing %%. The %% operator therefore fell through to the default genMultilineInfixExpr path, which always puts the operator at the start of the next line.

Fix

Add "%%" to noBreakInfixOps. When the expression is too long, the %% operator stays at the end of the LHS line and the RHS is indented:

let a =
    (if aVeryLongVariableNameThatForceLineBreaking = 0 then
         1
     else
         -1) %% 4   // ← valid: %% is on the same line as its left operand

This is identical to the existing behaviour for %, =, <, and >.

Test Status

• ✅ Build succeeded (dotnet build src/Fantomas.Core/Fantomas.Core.fsproj)
• ✅ All 2738 tests pass, 7 skipped (dotnet test src/Fantomas.Core.Tests/)
• ✅ New regression test added: custom double-percent operator stays on same line as lhs when expression is too long, 2107 in OperatorTests.fs
• ✅ Fixed output verified as valid F# (compiles and runs correctly)

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[nojaf]

/repo-assist you did not add a changelog entry here. Please open a new PR for this.
See https://github.com/fsprojects/fantomas/blob/main/.claude/skills/fantomas-issue/SKILL.md#7-update-changelogmd

[github-actions]

Pull request created: #3291

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