Gate native RDMA, cuObject, and cuobjclient implementations behind the
rdma build tag while keeping fallback stubs available for standard builds.
Preserve the separate cuobjclient_host configuration, clarify unsupported
platform errors, and update Makefile RDMA targets to pass the required tags
and disable VCS stamping.
Co-authored-by: Ben McClelland <ben.mcclelland@versity.com>
Cover the route error boundary with the real S3 server: the
shared serializer keeps status and body for wrapped S3 errors,
raw fiber errors stay 500, and the platform stub answers 501.
The stub-answer classifier moves next to the shared marker type
in the same commit so every build answers 501 at the point the
test first runs, and the general CI workflow builds the
session-server archive before go test, which the Linux link of
this package now requires.