Fix: CT parameter name uniqueness in CancellationToken overload generation

[Copilot]

If an OpenAPI spec defines a parameter named cancellationToken, the generated overload would emit two parameters with the same name — one from the spec and one for the injected CancellationToken.

Changes

• Track used parameter names: The parameter-fold now returns the usedNames set alongside providedParameters instead of discarding it with |> snd
• Unique CT parameter naming: The injected CT parameter resolves to the first unused name in the sequence cancellationToken → cancellationToken1 → cancellationToken2 → …, matching the existing UniqueNameGenerator convention
• Position-based extraction unaffected: invokeCode already extracts the CT argument by position (last in args), so it correctly handles the CT regardless of the name chosen

let usedNames, providedParameters =
    ((Set.empty, []), orderedParameters)
    ||> List.fold(fun (names, parameters) current -> ...)
    |> fun (names, ps) -> names, List.rev ps

let ctParamName =
    Seq.initInfinite(fun i -> if i = 0 then "cancellationToken" else $"cancellationToken{i}")
    |> Seq.find(fun n -> not(Set.contains n usedNames))

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