[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/client.go

@@ -21,6 +21,10 @@ type Client interface {
 	// is not eligible for session extension.
 	ExtendAuthSession(sysInfo *system.Info, jwtToken string) (*proto.ExtendAuthSessionResponse, error)
 	GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlow, error)
+	// GetPKCEAuthorizationFlow returns the PKCE authorization flow information.
+	//
+	// sessionExtend marks the flow as renewing an existing peer's session rather
+	// than logging one in. See PKCEAuthorizationFlowRequest.
 	GetPKCEAuthorizationFlow(sessionExtend bool) (*proto.PKCEAuthorizationFlow, error)
 	GetServerURL() string
 	// IsHealthy returns the current connection status without blocking.

shared/management/client/client_test.go

@@ -614,9 +614,7 @@ func Test_GetPKCEAuthorizationFlow(t *testing.T) {
 	}
 
 	flowInfo, err := client.GetPKCEAuthorizationFlow(true)
-	if err != nil {
-		t.Error("error while retrieving pkce auth flow information")
-	}
+	require.NoError(t, err)
 
 	assert.True(t, gotRequest.GetSessionExtend(), "session extend should reach the server")
 

shared/management/client/grpc.go

@@ -738,7 +738,7 @@ func (c *GrpcClient) GetDeviceAuthorizationFlow() (*proto.DeviceAuthorizationFlo
 	return flowInfoResp, nil
 }
 
-// GetPKCEAuthorizationFlow returns a pkce authorization flow information.
+// GetPKCEAuthorizationFlow returns the PKCE authorization flow information.
 // It also takes care of encrypting and decrypting messages.
 //
 // sessionExtend tells the server the flow will renew an existing peer's session

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