Don't allow gitops to disable gitops mode (#26794)

For #26743

This PR fixes an issue where using `fleetctl gitops` unsets the "gitops
mode" settings in the UI. The code which prepares the config spec to
send to the "modify config" endpoint deliberately copies over the
current app settings for gitops mode to facilitate this. I updated an
existing test to verify the new behavior.

cmd/fleetctl/gitops_test.go

@@ -234,7 +234,13 @@ func TestGitOpsBasicGlobalPremium(t *testing.T) {
 	// Mock appConfig
 	savedAppConfig := &fleet.AppConfig{}
 	ds.AppConfigFunc = func(ctx context.Context) (*fleet.AppConfig, error) {
-		return &fleet.AppConfig{}, nil
+		return &fleet.AppConfig{
+			// Set a GitOps UI mode to verify that applying GitOps config won't overwrite it.
+			UIGitOpsMode: fleet.UIGitOpsModeConfig{
+				GitopsModeEnabled: true,
+				RepositoryURL:     "https://didsomeonesaygitops.biz",
+			},
+		}, nil
 	}
 	ds.SaveAppConfigFunc = func(ctx context.Context, config *fleet.AppConfig) error {
 		savedAppConfig = config
@@ -326,6 +332,9 @@ software:
 	assert.Empty(t, enrolledSecrets)
 	assert.True(t, savedAppConfig.Integrations.NDESSCEPProxy.Valid)
 	assert.Equal(t, "https://ndes.example.com/scep", savedAppConfig.Integrations.NDESSCEPProxy.Value.URL)
+	// GitOps should not overwrite GitOps UI Mode.
+	assert.Equal(t, savedAppConfig.UIGitOpsMode.GitopsModeEnabled, true)
+	assert.Equal(t, savedAppConfig.UIGitOpsMode.RepositoryURL, "https://didsomeonesaygitops.biz")
 }
 
 func TestGitOpsBasicTeam(t *testing.T) {

server/service/client.go

@@ -574,6 +574,14 @@ func (c *Client) ApplyGroup(
 			specs.AppConfig.(map[string]interface{})["yara_rules"] = rulePayloads
 		}
 
+		// Keep any existing GitOps mode config rather than attempting to set via GitOps.
+		if appconfig != nil {
+			specs.AppConfig.(map[string]interface{})["gitops"] = fleet.UIGitOpsModeConfig{
+				GitopsModeEnabled: appconfig.UIGitOpsMode.GitopsModeEnabled,
+				RepositoryURL:     appconfig.UIGitOpsMode.RepositoryURL,
+			}
+		}
+
 		if err := c.ApplyAppConfig(specs.AppConfig, opts.ApplySpecOptions); err != nil {
 			return nil, nil, nil, nil, fmt.Errorf("applying fleet config: %w", err)
 		}