RtmaClient was the only weather client without a Req.Test plug seam —
added the standard req_options() helper matching NarrClient/HrrrClient/
etc. so its HTTP calls can be stubbed in test.
17 characterization tests: URL construction, idx fetch 404/503, hour
truncation, malformed idx, range-GET error propagation, worker's
existing-row short-circuit, client-error propagation. GRIB2 byte
parsing paths deferred to the wgrib2 coverage task.
