`.userAuthenticationRequired`, `.clientCertificateRequired`, `.clientCertificateRejected` fall to `GenericError` despite closer variants existing

> Split out from Automattic/wordpress-rs#1497 — *Swift executor URLSession error audit*. Section B — Unchecked URLError codes we could classify better.
>
> 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.** `HttpAuthenticationRequiredError { hostname, method: Option }` already exists with an optional method, so `.userAuthenticationRequired` (authenticating proxies) can map there with `method: nil`. The mTLS codes could take either the auth variants or `InvalidSslError(.genericSslError)` — either beats `GenericError`. Occurrence is limited to authenticating proxies and mTLS-fronted corporate WordPress — rare but real.

Start in native/swift/Sources/wordpress-api/SafeRequestExecutor.swift and inspect the handling of .userAuthenticationRequired, .clientCertificateRequired, and .clientCertificateRejected. Use the existing HttpAuthenticationRequiredError and InvalidSslError variants as the reference, then confirm all three codes no longer fall through to GenericError.

Written by the indexing model from the issue text.