Fix WG client Windows CI regressions

[moloch--]

Summary

• preserve fast connection spinner status updates
• close gRPC connections before transport teardown on Windows-safe paths
• reuse only just-closed idle WireGuard tunnels so fast reconnects work without sharing active tunnels

Verification

• GOCACHE=/tmp/sliver-go-build-clean ./go-tests.sh --unit-only
• Windows repro on proxmox-win11: ./server/console passed under go-tests.ps1 with the refined idle-tunnel reuse logic

Supersedes #2234 with the minimal change set from the pre-fix PR head.