## Proposed changes
- Validate Cloud server configuration before initializing TxnKv,
MetaService, Recycler, or background threads.
- Keep start-time validation as a defensive check.
- Add OSS starter unit coverage for plaintext, unsupported TLS, and
null-server configurations.
## Problem
Cloud server validation currently runs after MetaService, Recycler, and
the Recycler logging thread have started. If validation rejects the
configuration, `main` returns through partially initialized resources,
and the joinable Recycler thread can terminate the process during stack
unwinding.
