[client] Address review comments on PKCE session extend flow

Fail the PKCE authorization flow test on request error instead of
continuing into a nil dereference, and make the godoc comments on the
touched exported symbols identifier-leading full sentences.

shared/management/client/mock.go

@@ -80,6 +80,7 @@ func (m *MockClient) GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlo
 	return m.GetDeviceAuthorizationFlowFunc()
 }
 
+// GetPKCEAuthorizationFlow returns the PKCE authorization flow information.
 func (m *MockClient) GetPKCEAuthorizationFlow(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error) {
 	if m.GetPKCEAuthorizationFlowFunc == nil {
 		return nil, nil