A Swift Task cancelled before the URLSession task is registered runs the request to completion

> Split out from Automattic/wordpress-rs#1497 — *Swift executor URLSession error audit*. Section E — Cancellation gaps.
>
> File references are to `native/swift/Sources/wordpress-api/SafeRequestExecutor.swift` unless noted; `:NNN` line numbers were verified against `fix/converge-executor-error-classification`.

**Severity: medium.** In both `perform` implementations, `withTaskCancellationHandler`'s `onCancel` calls `cancellation.cancel()`, which nils a still-unset `_task`; `TaskCancellation` has no cancelled latch, and `withTaskCancellationHandler` invokes `onCancel` immediately when the surrounding Task is already cancelled — before the continuation body creates the URLSession task. A pre-cancelled or racing cancellation therefore cancels nothing: the request executes fully (a cancelled POST/DELETE still sends the mutation) and the caller receives success instead of `CancellationError`. Fix: latch `cancel()` and cancel any task assigned after the fact, and/or check `Task.isCancelled` before resuming.

Start in native/swift/Sources/wordpress-api/SafeRequestExecutor.swift, focusing on both perform implementations and TaskCancellation. Trace pre-cancelled and racing cancellation through the continuation and URLSession task registration; done means cancellation prevents the request from completing and the caller receives CancellationError.

Written by the indexing model from the issue text.