Add context isolation for peer connections

Use core.ToBackgroundDetachedContext to prevent all peer connections from
being cancelled when the original request context is cancelled. This ensures
peer connections remain independent and stable.

Tests pass, no security issues found.

Co-authored-by: RPRX <63339210+RPRX@users.noreply.github.com>

proxy/wireguard/client.go

@@ -129,7 +129,7 @@ func (h *Handler) processWireGuard(ctx context.Context, dialer internet.Dialer)
 			},
 			workers: workers,
 		},
-		ctx:      ctx,
+		ctx:      core.ToBackgroundDetachedContext(ctx),
 		dialer:   dialer,
 		reserved: h.conf.Reserved,
 	}