rdma: test RC route errors through the production server

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.

.github/workflows/go.yml

@@ -24,6 +24,9 @@ jobs:
       run: |
         go get -v -t -d ./...
 
+    - name: Build RDMA session server archive
+      run: make rdma/librcserver.a
+
     - name: Test
       run: go test -coverprofile profile.txt -race -v -timeout 30s -tags=github ./...