What?

Part of #45668.

PR makes the following improvements to the Post Author block:

• Fetch data on demand. Avoid HTTP requests when control isn't rendered.
• Allow searching for more than 100 users, without regressing performance.
• Display data for all users with the right capabilities. Previously, it was only working for admin users.
• Ensure the current author is included in the list.
• Removes the condition for switching between Select and Combobox. I think the latter has better UX for similar settings.

Why?

The general plan is to consolidate similar author selection controls across the blocks. I'm avoiding early abstraction of AuthorCombobox as suggested in the issue, and here's why:

• The @wordpress/block-editor can't depend on the core-data package.
• The @wordpress/editor is a high-level package and can only be used by edit-post/edit-site packages.
• Best option is to have this as a shared component in the block-library package, which I'll handle as the last step of Unify user selection controls across the blocks #45668.

How?

The refactoring closely mimics the PostAuthor component logic from the editor package.

Testing Instructions

1. Open a post or page.
2. Insert the Post Author block.
3. Confirm that author selection continues to work as before for admin users.
4. Repeat the same step with the editor user role.
5. Confirm that the author selection settings are now visible for this role.
6. Repeat the test for the author user role.
7. The author selection should be hidden for this role.

Testing Instructions for Keyboard

Same.

Screenshots or screencast