add openvpn input validation

[cachebag]

part of #288.

changes

add validate_openvpn_config(config: &OpenVpnConfig) in util/validation.rs:

• connection name validation (reuse validate_connection_name)
• remote server non-empty
• port range check if set
• auth-type-specific required field checks (username for password, certs for tls)
• cert paths are non-empty strings (don't check file existence — that's NM's job)
• dns validation (reuse existing)
• mtu range check (reuse existing)

refactor validate_vpn_credentials to dispatch on VpnType instead of assuming wireguard.

tests

unit tests for each validation path — missing fields, invalid ports, auth type mismatches.