Refactor vpn.rs to support multiple vpn types

[stoutes]

Summary

Part of #288, closes #293.

Refactors core/vpn.rs to replace hardcoded WireGuard type checks with a
shared detect_vpn_type() helper that understands both WireGuard and OpenVPN
connection types, making each function ready to handle multiple VPN backends.

Changes

api/models/vpn.rs

• Added OpenVpn variant to VpnType enum

core/vpn.rs

• Added detect_vpn_type() helper — detects WireGuard (connection.type == "wireguard")
  and OpenVPN (connection.type == "vpn" + vpn.service-type == "org.freedesktop.NetworkManager.openvpn")
  from a settings map, replacing all scattered inline string checks
• connect_vpn: dispatches on creds.vpn_type to build_wireguard_connection or returns
  VpnFailed for unsupported types — full impl VpnConfig signature blocked on VpnConfig
  trait from openvpn support #288
• disconnect_vpn: replaced hardcoded connection.type == "wireguard" check with detect_vpn_type
• list_vpn_connections: expanded active and saved connection filters to detect both types via
  detect_vpn_type; vpn_type on returned VpnConnection now reflects actual detected type
  instead of always being VpnType::WireGuard
• get_vpn_info: replaced hardcoded type check with detect_vpn_type; gateway extraction
  branches on vpn_type — WireGuard reads from wireguard.peers, OpenVPN reads from
  vpn.data["remote"]; IP config extraction unchanged
• forget_vpn: replaced hardcoded type check with detect_vpn_type, flattened
  if let Some(conn_sec) nesting

api/network_manager.rs

• Simplified connect_vpn to delegate fully to core::vpn::connect_vpn, removed redundant
  build_wireguard_connection call and unused opts parameter

Blocked on #288

• build_openvpn_connection — OpenVPN connections will hit the VpnFailed stub until this lands
• VpnConfig trait — connect_vpn signature cannot be updated to impl VpnConfig until the
  trait exists

[cachebag]

@stoutes Could you please drop these other commits from the branch? I'd prefer you just keep the changes you made. It looks like you branched off of master when it should have been off of dev-openvpn.

Additionally, please clean up your commit messages. This is not ideal

[cachebag]

@stoutes You're gonna want to run git rebase -i HEAD~11 and drop the 6 commits that come before yours.

[stoutes]

@stoutes You're gonna want to run git rebase -i HEAD~11 and drop the 6 commits that come before yours.

Yeah not sure how that happened....weird

[cachebag]

Nice job, really appreciate your help on this!

[cachebag]

Just to note @stoutes for the conflcits, accept all incoming changes. We don't want gateway in our VpnConfig trait

[cachebag]

LGTM