fix: guard segment update against no-op full PUT

segment update lacked the "nothing to update" guard its sibling
commands have, so a no-flag invocation fetched the segment and
re-PUT it unchanged — an audit-log entry and rule-tree overwrite
for a no-op. Fail fast before any network call instead.

beep boop

Author: khvn26

internal/cmd/cmd_test.go

@@ -3504,6 +3504,18 @@ func TestSegmentCreate(t *testing.T) {
 }
 
 func TestSegmentUpdate(t *testing.T) {
+	t.Run("nothing to update errors without touching the segment", func(t *testing.T) {
+		f := flagUpdateEnv(t)
+		_, err := run("", "segment", "update", "us-adults")
+		var ue *usageError
+		if !errors.As(err, &ue) || !strings.Contains(err.Error(), "nothing to update") {
+			t.Errorf("err = %v, want a usage error", err)
+		}
+		if f.lastSegmentBody != nil {
+			t.Errorf("segment was PUT despite no changes: %+v", f.lastSegmentBody)
+		}
+	})
+
 	t.Run("keeps rules when only description changes", func(t *testing.T) {
 		f := flagUpdateEnv(t)
 		if _, err := run("", "segment", "update", "us-adults", "--description", "new desc"); err != nil {

internal/cmd/segment.go

@@ -188,6 +188,9 @@ var segmentUpdateCmd = &cobra.Command{
   flagsmith segment update beta-users --description "Updated cohort"`,
 	Args: cobra.ExactArgs(1),
 	RunE: func(cmd *cobra.Command, args []string) error {
+		if !cmd.Flags().Changed("rules") && !cmd.Flags().Changed("description") && !cmd.Flags().Changed("feature") {
+			return usageErrorf("nothing to update")
+		}
 		cred, projectID, err := projectScopedContext(cmd)
 		if err != nil {
 			return err