Comment List Tables:

* Declare `$extra_items` property
* Since `->get_column_info()` overrides its parent's method, it doesn't need to set `_column_headers`, which is an undeclared property that is only used for caching the lookup after the first run in the parent class, which isn't called in the child class.
* `$_args` in `WP_List_Table` has to be `protected` for `WP_Post_Comments_List_Table` to legally access it. `$_args` is awkward as is and should probably be only available via a getter.

See #30799.

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