SDK 1.153.1 always defaults to Spanner when backend_config is omitted,
and crashes when it's provided. Use REST POST directly with
vectorDbConfig.ragManagedDb set to force serverless.
SDK 1.153.1 has a bug in set_backend_config where it calls .CopyFrom()
on RagManagedDb which is not a real protobuf message.
The project-level ragEngineConfig PATCH already sets basic tier,
so passing backend_config is unnecessary and breaks corpus creation.
- Pass RagVectorDbConfig with RagManagedDbConfig basic tier explicitly
to avoid Spanner allowlist errors on new projects
- Add pre-flight ensure_serverless_engine_config() via REST PATCH
- Cleaner error messages with actionable hints
- Idempotent: skips creation if corpus already exists
