atunnel: refresh client CA pool per handshake

In `internal/atunnel/server.go` the client CA pool is built once at startup, while leaf certificates are reloaded per connection. Because substrate certs expire within ~1–2 days, a frozen CA pool means the server will eventually reject valid peers (or keep trusting a rotated-out CA) until it restarts.

Implement tls.Config.GetConfigForClient so the client CA pool is re-read on each handshake, mirroring the existing per-connection leaf cert reload.

source: https://github.com/agent-substrate/substrate/pull/559#discussion_r3687538721