Fix CancellationToken parameter ordering and name collision in v3 OperationCompiler

[Copilot]

Two bugs in the CT overload generation: CT was appended after optional parameters (invalid in .NET — required params must precede optional), and the CT parameter name was always cancellationToken with no collision check against generated OpenAPI parameter names.

Changes

• Parameter ordering: Split parameter building into two passes (required, then optional) with the used-name set threaded between them. CT is now inserted at ctArgIndex = requiredParamCount, giving the signature: (required...) → CT → (optional...).

• Name uniqueness: Collect all generated parameter names into a set before creating the CT param; fall back to cancellationToken1, cancellationToken2, etc. on collision.

• invokeCode extraction: CT is now extracted by List.item ctArgIndex allArgs (index captured during parameter building) rather than by peeling off the last argument, which broke under the new ordering.

Before / After signature example

// Before: CT appended last — after optional params
MyMethod(requiredA: string, ?optB: int, cancellationToken: CancellationToken)

// After: CT sits between required and optional
MyMethod(requiredA: string, cancellationToken: CancellationToken, ?optB: int)

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