Improved fleetctl gitops error message when trying to change team name to a team that already exists. (#21214)

#21104 
# Checklist for submitter


- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/Committing-Changes.md#changes-files)
for more information.
- [x] Added/updated tests
- [x] Manual QA for all new/changed functionality

changes/21104-gitops-team-conflict

@@ -0,0 +1 @@
+Improved fleetctl gitops error message when trying to change team name to a team that already exists.

cmd/fleetctl/gitops_test.go

@@ -665,6 +665,9 @@ func TestFullTeamGitOps(t *testing.T) {
 	// Team
 	var savedTeam *fleet.Team
 	ds.TeamByNameFunc = func(ctx context.Context, name string) (*fleet.Team, error) {
+		if name == "Conflict" {
+			return &fleet.Team{}, nil
+		}
 		if savedTeam != nil && savedTeam.Name == name {
 			return savedTeam, nil
 		}
@@ -825,7 +828,15 @@ func TestFullTeamGitOps(t *testing.T) {
 	assert.Equal(t, newTeamName, savedTeam.Name)
 	assert.Equal(t, baseFilename, *savedTeam.Filename)
 
+	// Try to change team name again, but this time the new name conflicts with an existing team
+	t.Setenv("TEST_TEAM_NAME", "Conflict")
+	_, err = runAppNoChecks([]string{"gitops", "-f", file, "--dry-run"})
+	assert.ErrorContains(t, err, "team name already exists")
+	_, err = runAppNoChecks([]string{"gitops", "-f", file})
+	assert.ErrorContains(t, err, "team name already exists")
+
 	// Now clear the settings
+	t.Setenv("TEST_TEAM_NAME", newTeamName)
 	tmpFile, err := os.CreateTemp(t.TempDir(), "*.yml")
 	require.NoError(t, err)
 	secret := "TestSecret"

ee/server/service/teams.go

@@ -868,6 +868,20 @@ func (svc *Service) ApplyTeamSpecs(ctx context.Context, specs []*fleet.TeamSpec,
 			if err != nil && !fleet.IsNotFound(err) {
 				return nil, err
 			}
+			if team != nil && team.Name != spec.Name {
+				// If user is trying to change team name, check that the new name is not already taken.
+				_, err = svc.ds.TeamByName(ctx, spec.Name)
+				switch {
+				case err == nil:
+					return nil, fleet.NewInvalidArgumentError("name",
+						fmt.Sprintf("cannot change team name from '%s' (filename: %s) to '%s' because team name already exists", team.Name,
+							*spec.Filename, spec.Name))
+				case fleet.IsNotFound(err):
+					// OK
+				default:
+					return nil, err
+				}
+			}
 		}
 		var create bool
 		if team == nil {