UrlLib: expose HTTP status reason phrase (statusText) #193

Follow-up from the native fetch() polyfill review (#188).

UrlLib currently exposes only the numeric HTTP status code. To provide Response.statusText, the Fetch polyfill added a hardcoded code→text table (StatusText in Polyfills/Fetch/Source/Fetch.cpp) that returns "" for any code not in the table.

This mapping belongs in UrlLib instead:

• Expose the real reason phrase from the transport where the wire carries one (HTTP/1.x).
• Fall back to a code→text table for HTTP/2+ where the status line carries no reason phrase.

Doing this in UrlLib would also let XMLHttpRequest — which currently has no statusText — expose one for free, and remove the per-polyfill table.