Expand ability name validation from exactly 2 segments (`namespace/ability`) to 2-4 segments, enabling names like `my-plugin/resource/find` and `my-plugin/resource/sub/find`.
This allows plugins to organize abilities into logical resource groups. The validation regex changes from `/^[a-z0-9-]+\/[a-z0-9-]+$/` to `/^[a-z0-9-]+(?:\/[a-z0-9-]+){1,3}$/`, which accepts the first segment plus 1-3 additional slash-delimited segments.
Updates the validation regex, error messages, docblocks, and adds corresponding unit and REST API tests.
Props jorgefilipecosta, justlevine, jorbin.
Fixes#64596.
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