version: error on invalid --output instead of silent success

Invalid values like --output=yml previously exited 0 with no output.
Return a clear error matching kubectl's --output validation.

Signed-off-by: dpacgdm <dpac.gdm@gmail.com>

Author: dpacgdm

kustomize/commands/version/version.go

@@ -81,6 +81,8 @@ func (o *Options) Run() error {
 			return errors.WrapPrefixf(err, "marshalling provenance to json")
 		}
 		fmt.Fprintln(o.Writer, string(marshalled))
+	default:
+		return fmt.Errorf("--output must be 'yaml' or 'json'")
 	}
 	return nil
 }

kustomize/commands/version/version_test.go

@@ -0,0 +1,26 @@
+// Copyright 2026 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package version
+
+import (
+	"bytes"
+	"strings"
+	"testing"
+)
+
+func TestRunInvalidOutput(t *testing.T) {
+	var buf bytes.Buffer
+	o := NewOptions(&buf)
+	o.Output = "yml"
+	err := o.Run()
+	if err == nil {
+		t.Fatal("expected error for invalid --output")
+	}
+	if !strings.Contains(err.Error(), "--output must be 'yaml' or 'json'") {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if buf.Len() != 0 {
+		t.Fatalf("expected no output on invalid --output, got %q", buf.String())
+	}
+}