Declare sampling capability in ServerCapabilities

[jserv]

Summary

ServerCapabilities omits the sampling field. Clients must probe or timeout to discover whether sampling is supported.

Details

• Fix: add sampling: Option<SamplingCapability> to ServerCapabilities in types.rs. Set Some(...) in sync transport (which supports sampling), None in async transport.
• This avoids unnecessary client probe/timeout on feature discovery.
• Gate: E2E test verifies capabilities.sampling is present in the initialize response.