When a column has an editor, `setRenderer` needs to apply the editor
renderer again on top of the new regular renderer. This logic duplicated
most of `setupColumnEditor`: rendering the editor renderer and
registering its data generator.

`setupColumnEditor` now skips creating the `EditorRenderer` when it
already exists, so `setRenderer` can call it instead of repeating the
same steps.

Extracted from https://github.com/vaadin/flow-components/pull/6798

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🤖 Generated with Claude Code

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>