v6: unit test harness — in-process gRPC fakes

[Pandys]

• In-process gRPC servers implementing generated servicers with canned logic; exercised through the FULL client stack (interceptors, retries, codec) — no network
• Coverage targets: retry behaviour on UNAVAILABLE, error mapping incl. NotSupportedError, location metadata on every call type (unary/server-stream/client-stream), pagination iterator, DataFrame round-trip, slug resolution + cache invalidation
• Contract smoke test against a staging cluster, gated behind an env flag, run only in the proto-sync workflow