`WpNetworkResponse.init` uses `preconditionFailure` where the throwing error path already exists

> Split out from Automattic/wordpress-rs#1497 — *Swift executor URLSession error audit*. Section D — Crash instead of classified error.
>
> 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: low.** `guard let response = response as? HTTPURLResponse else { preconditionFailure(...) }` (`Extensions.swift:17`). The initializer is already `throws` and is called inside `perform()`'s do-block, so `throw URLError(.badServerResponse)` degrades gracefully to `.genericError` today. Verified effectively unreachable for http(s) loads on Darwin and Linux — defense-in-depth, retained because the fix is one line. (The same verification cleared the completion-handler `data!`/`response!` force-unwraps on both platforms: no finding there.)

Start in native/swift/Sources/wordpress-api/SafeRequestExecutor.swift and inspect the WpNetworkResponse initializer, especially the guard at Extensions.swift:17 and its throwing call path from perform(). Done means the invalid response path no longer terminates the process and is reported through the existing classified error flow.

Written by the indexing model from the issue text.