`.unsupportedURL` is not grouped with `.badURL`

> 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.** `errorIsNonExistentSiteError` includes `.badURL` ("kept for completeness" per its own comment, `:248-265`) but omits its sibling `.unsupportedURL` (-1002), which therefore falls to `GenericError`. The two codes represent the same user situation — the entered site URL cannot be requested — and the branch comment's rationale applies verbatim. The rustdoc note documenting the `badURL` grouping should mention it too.

Open native/swift/Sources/wordpress-api/SafeRequestExecutor.swift and inspect errorIsNonExistentSiteError around lines 248-265, including its branch comment and rustdoc note. Done means .unsupportedURL is grouped with .badURL and the documentation explicitly mentions both codes.

Written by the indexing model from the issue text.