rpc : log the actual network transport in use
Add log messages at the transport decision points so the user can tell
whether an RPC connection is actually using RDMA or falling back to TCP.
- rdma_probe(): log each reason for failing to set up RDMA instead of
silently returning false
- update_caps(): log 'RDMA transport active' on success and
'RDMA not negotiated, staying on TCP' when the peer advertises no caps
Assisted-by: llama.cpp:DeepSeek-4-Flash-0731
