Raise TCP claim send buffer before connect

[loriopatrick]

Summary

• Increase the TCP send buffer used for claim connections to 8 MiB before dialing the claim address.
• Switch claim connection setup to TcpSocket so the buffer can be configured prior to connect().
• Log buffer configuration failures while still attempting the connection.

Testing

• Not run
• Verified the change is limited to packages/agent_core/src/network/tcp/tcp_clients.rs
• Confirmed claim connection path now uses the new helper that sets the send buffer before connecting