fix(transfer): clean up failed RDMA handshakes

[xiaguan]

Summary

• make RC handshake completion roll back connecting state and pending QPs when completion fails
• validate remote memory metadata before committing connection state
• split RcSession worker-owned I/O state from the session handle so failed pending sessions can release worker/QP resources
• report RDMA fetch summary bandwidth as GB/s

Fixes #233.

Verification

• cargo fmt --check
• cargo test -p pegaflow-transfer
• cargo clippy --workspace --all-targets -- -D warnings
• pre-commit hooks during commit amend, including cargo test --release

Review

• toxic reviewer pass: ready for review