httpBatchLink joins same-tick calls into one request whose path is a
comma-separated list of procedure paths; the stub matched the joined
string against a single mock key, so any batched pair failed with 'No
mock for a.one,a.two'. It now answers one element per procedure, in
order. Error bodies are also superjson-serialized — raw error JSON made
the client throw 'Unable to transform response' instead of surfacing
the intended error. Regression tests added.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>