Allow selector-only template columns to enter edit mode

[Copilot]

TableViewTemplateColumn already supports EditingTemplateSelector when generating the editing element, but TableViewCell.IsReadOnly still treated template columns without EditingTemplate as non-editable. As a result, selector-only editing templates were blocked before the edit path could run.

• Read-only gate

  • Update TableViewCell.IsReadOnly so TableViewTemplateColumn is considered read-only only when both EditingTemplate and EditingTemplateSelector are unset.
  • Preserve existing behavior for table-level and column-level read-only settings.

• Regression coverage

  • Add a focused test for a TableViewCell bound to a TableViewTemplateColumn that sets only EditingTemplateSelector.
  • Verify the cell is not marked read-only in that configuration.

• Behavioral impact

  • Align the editability check with the existing editing-element generation logic.
  • Remove the need for the empty-DataTemplate workaround.

public bool IsReadOnly => TableView?.IsReadOnly is true
                          || Column is TableViewTemplateColumn
                          {
                              EditingTemplate: null,
                              EditingTemplateSelector: null
                          }
                          or { IsReadOnly: true };