[client] Remove endpoint address on peer disconnect, retain status for activity recording (#4228)

* When a peer disconnects, remove the endpoint address to avoid sending traffic to a non-existent address, but retain the status for the activity recorder.

client/internal/engine_test.go

@@ -105,6 +105,10 @@ type MockWGIface struct {
 	LastActivitiesFunc         func() map[string]monotime.Time
 }
 
+func (m *MockWGIface) RemoveEndpointAddress(_ string) error {
+	return nil
+}
+
 func (m *MockWGIface) FullStats() (*configurer.Stats, error) {
 	return nil, fmt.Errorf("not implemented")
 }