fix: complete NETCONF implementation in junos_config.rs

[adolago]

Summary

The JunOS network module (src/modules/network/junos_config.rs) has an incomplete NETCONF implementation. The execute_netconf_via_cli() method currently returns an empty string with a comment: "For now, we simulate NETCONF via CLI commands."

Issues

1. execute_netconf_via_cli() returns empty string — no actual NETCONF RPC execution
2. send_rpc_raw() has shell command construction that needs security review (lines ~437-443)
3. NETCONF over SSH subsystem not implemented

Acceptance criteria

• NETCONF RPC execution works via SSH subsystem or has a robust CLI fallback
• Shell command construction uses shell_escape for user-provided values
• Unit tests cover NETCONF operation paths
• Check mode properly supported

Priority

Medium — other network modules (ios_config, eos_config) are fully functional