Add missing translation keys for MTU/MSS Fix validation errors (#136)

* Initial plan

* fix: add missing translation keys for tun_mtu and mssfix validation

Co-authored-by: gsanchietti <[email protected]>

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: gsanchietti <[email protected]>

Author: Copilot