Add set-based label selector test and docs coverage for restore

Restore label selector filtering had only equality-based coverage;
set-based selectors were exercised nowhere in the repo, relying
entirely on apimachinery behavior. Add notin, in, and doesnotexist
(!key) cases to TestRestoreResourceFiltering and set-based parse
cases for the --selector CLI flag.

Also document restore usage of --selector in resource-filtering.md:
the notin phased-restore scenario (noting notin also matches
resources without the label key), restoring only unlabeled resources
via '!<key>', and the caveat that restore item action dependencies
bypass label selectors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Signed-off-by: Tiger Kaovilai <tkaovila@redhat.com>

pkg/cmd/util/flag/label_selector_test.go

@@ -24,6 +24,28 @@ func TestSetOfLabelSelector(t *testing.T) {
 	assert.True(t, str == "k1=v1,k2=v2" || str == "k2=v2,k2=v2")
 }
 
+func TestSetOfSetBasedLabelSelector(t *testing.T) {
+	selector := &LabelSelector{}
+	require.NoError(t, selector.Set("pr-label notin (1)"))
+	require.NotNil(t, selector.LabelSelector)
+	require.Len(t, selector.LabelSelector.MatchExpressions, 1)
+	req := selector.LabelSelector.MatchExpressions[0]
+	assert.Equal(t, "pr-label", req.Key)
+	assert.Equal(t, metav1.LabelSelectorOpNotIn, req.Operator)
+	assert.Equal(t, []string{"1"}, req.Values)
+}
+
+func TestSetOfDoesNotExistLabelSelector(t *testing.T) {
+	selector := &LabelSelector{}
+	require.NoError(t, selector.Set("!pr-label"))
+	require.NotNil(t, selector.LabelSelector)
+	require.Len(t, selector.LabelSelector.MatchExpressions, 1)
+	req := selector.LabelSelector.MatchExpressions[0]
+	assert.Equal(t, "pr-label", req.Key)
+	assert.Equal(t, metav1.LabelSelectorOpDoesNotExist, req.Operator)
+	assert.Empty(t, req.Values)
+}
+
 func TestTypeOfLabelSelector(t *testing.T) {
 	selector := &LabelSelector{}
 	assert.Equal(t, "labelSelector", selector.Type())