Reject malformed JSON-RPC messages instead of silently discarding them

[jserv]

Summary

Response-shaped messages (has id, no method) are silently dropped in the transport layer. Per JSON-RPC 2.0 spec, the server should return -32600 INVALID_REQUEST.

Details

• Messages with id but no method are silently discarded in both sync and async transports.
• Fix: return INVALID_REQUEST (-32600) for these messages. Keep silent discard only for messages with no id (stale sampling responses).
• Depends on: structured TransportError enum (makes the codec vs invalid-request distinction expressible without string-matching) and malformed protocol E2E tests.