fix(export): reject unsupported -o format on empty resource lists

pkg/cmd/consumer/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/consumergroup/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/globalrule/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/pluginconfig/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/proto/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/route/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/service/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/ssl/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/streamroute/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmd/upstream/export/export.go

@@ -50,6 +50,10 @@ func NewCmdExport(f *cmd.Factory) *cobra.Command {
 }
 
 func exportRun(opts *Options) error {
+	if err := cmdutil.ValidateExportFormat(opts.Output); err != nil {
+		return err
+	}
+
 	cfg, err := opts.Config()
 	if err != nil {
 		return err

pkg/cmdutil/exporter.go

@@ -23,6 +23,22 @@ func NewExporter(format string, writer io.Writer) *Exporter {
 	}
 }
 
+// ValidateExportFormat returns an error if format is not one of the formats
+// that Exporter.Write accepts. The empty string is treated as valid because
+// every export command defaults it to "yaml" later in its run.
+//
+// Callers should invoke this before any work that could short-circuit the
+// run (e.g. an "empty collection" early return), so an invalid -o flag is
+// rejected consistently regardless of the result set size.
+func ValidateExportFormat(format string) error {
+	switch format {
+	case "", "json", "yaml":
+		return nil
+	default:
+		return fmt.Errorf("unsupported output format: %s", format)
+	}
+}
+
 // Write formats and writes the given data.
 func (e *Exporter) Write(data interface{}) error {
 	switch e.format {

pkg/cmdutil/exporter_test.go

@@ -70,3 +70,25 @@ func TestExporter_JSONPrettyPrinted(t *testing.T) {
 	// Verify pretty printing (indented)
 	assert.Contains(t, buf.String(), "  \"key\"")
 }
+
+func TestValidateExportFormat(t *testing.T) {
+	t.Run("accepts json", func(t *testing.T) {
+		require.NoError(t, ValidateExportFormat("json"))
+	})
+	t.Run("accepts yaml", func(t *testing.T) {
+		require.NoError(t, ValidateExportFormat("yaml"))
+	})
+	t.Run("accepts empty (callers default later)", func(t *testing.T) {
+		require.NoError(t, ValidateExportFormat(""))
+	})
+	t.Run("rejects table", func(t *testing.T) {
+		err := ValidateExportFormat("table")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "unsupported output format: table")
+	})
+	t.Run("rejects garbage", func(t *testing.T) {
+		err := ValidateExportFormat("garbage-format")
+		require.Error(t, err)
+		assert.Contains(t, err.Error(), "unsupported output format: garbage-format")
+	})
+}