Fix ExcludePostsControl fetching all posts

[PatelUtkarsh]

Fixes #157

Props to @trevormills-xwp for the PostPickerControl refactor, debounce, loading states, and backwards compat migration.

Problem

ExcludePostsControl calls getEntityRecords with per_page: -1. The REST API middleware splits this into paginated requests of 100. On sites with thousands of posts, these requests pile up, hang the block editor, and block post saving and autosaves.

Fix

Refactors both ExcludePostsControl and PostIncludeControls into a shared PostPickerControl component that:

• Replaces per_page: -1 with per_page: 10 with debounced search-as-you-type (500ms)
• Adds search_columns: post_title so REST results match what FormTokenField filters client-side
• Adds _fields: id,title to keep responses lightweight
• Skips search fetch when input is empty
• Shows loading state ("Searching...") and idle hint ("Type to search by title")
• Deduplicates suggestions via Set to prevent FormTokenField issues
• Resets search term on token selection

Backwards compatibility:

• exclude_posts attribute format changed from [id, id] to [{id, title}, ...] to match include_posts
• Auto-migration: old format is detected and converted on load via useSelect + useEffect
• PHP Exclude_Posts trait updated to handle numeric, {id, title}, and plain array formats
• Unit tests added for both legacy and new formats

Other changes:

• defined('ABSPATH') || exit changed to if block in PHP files for phpunit compatibility

Testing

1. Create a Post on a site with thousands of posts
2. Insert an AQL block, click "Start blank", pick a variation
3. Switch Query Type to Custom
4. Editor should stay responsive. Save draft should complete.
5. In "Posts to Exclude", search by title. Suggestions should appear with debounce.
6. Add a post, save, reload - confirm it persists.
7. Search and add a second post - confirm both remain after save.
8. Test with a post that has old-format exclude_posts (array of IDs) - confirm auto-migration to {id, title} format.
9. Test "Posts to Include" - same search behavior should work.
10. Network tab should show single wp/v2/posts requests with per_page=10, not paginated chains.