Worker-owned httr2 and curl implementation of the Rho HTTP contract

rho_httr2_http_client() constructs an HttpClient owned by a compute worker. Complete requests use httr2. For a streaming request, the worker uses curl's native multi event loop, captures the final response head before relaying its first body chunk, and sends typed head, raw chunk, completion, or error values over a private localhost NNG socket. The calling process receives those values through RhoTask and RhoStream without blocking its R event loop.

Details

Complete requests also execute through the selected compute backend. Closing a body stream or client closes its NNG socket and cancels the owning compute task. The default compute backend is mirai; callers may supply another RhoComputeBackend that preserves the same call specification semantics. rho.http::rho_http_open_execution() returns RhoHttpWorkerOpen for this client because connection setup and receipt of the response head occur in that selected worker.