tolowerequalfold: avoid false positives on ToLower(x) == x / != x case-detection idioms

[Copilot]

tolowerequalfold was flagging comparisons where one side is strings.ToLower/ToUpper and the other is the same operand, e.g. strings.ToLower(x) != x. Those are case-detection checks, not case-insensitive equality checks, and rewriting to strings.EqualFold can produce incorrect logic.

• Analyzer precision: self-comparison guard

  • Added a guard in pkg/linters/tolowerequalfold/tolowerequalfold.go to skip diagnostics when:
    • one side is strings.ToLower/ToUpper(E), and
    • the other side is the same identifier E (matched via types.Object identity).
  • Refactored call detection with caseConvArg(...) and reused it from isCaseConvCall(...).

• Fixture coverage for the missed idiom

  • Extended pkg/linters/tolowerequalfold/testdata/src/tolowerequalfold/tolowerequalfold.go okExamples with:
    • strings.ToLower(name) == name
    • strings.ToLower(name) != name
  • Existing flagged examples remain unchanged (e.g. comparisons against literals / different expressions).

if arg, ok := caseConvArg(expr.X); ok && sameOperand(pass, arg, expr.Y) {
    return // case-detection idiom: do not suggest EqualFold
}
if arg, ok := caseConvArg(expr.Y); ok && sameOperand(pass, expr.X, arg) {
    return
}
if isCaseConvCall(expr.X) || isCaseConvCall(expr.Y) {
    pass.ReportRangef(expr, "use strings.EqualFold ...")
}