Closes #7650

Problem

WireGuard mode can fail to start with an "address already in use" error, but the message lacks the usual hint suggesting to pick a different port.

Solution

Extend the listener startup error handling so that WireGuard port-conflict exceptions (including non-OSError exceptions from the Rust backend) get wrapped into an OSError with the same helpful hint text used in other modes.

Testing

Added a regression test for the WireGuard port-conflict case.