Page List: Parent control query limit is too small

[snehapatil2001]

What?

Closes #69608

Increases the Page List block parent control query limit from 100 to 500 pages.## Why?

The Page List block currently cannot be used effectively on websites with more than 100 pages because the parent control dropdown is limited to showing only 100 pages. This makes it impossible for users to select parent pages that are deeper in the hierarchy or later in the alphabetical listing when there are more than 100 pages on the site.

How?

This PR updates the MAX_PAGE_COUNT constant in the Page List block's edit.js file, increasing the limit from 100 to 500 pages. This allows the parent control dropdown to load and display up to 500 pages, making the block usable on larger websites.

Testing Instructions

1. Create or use a WordPress site with more than 100 pages
2. Edit a page and insert a Page List block
3. Click the "Parent Page" dropdown in the block settings
4. Verify that you can now see and select pages beyond the first 100 pages

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Co-authored-by: snehapatil2001 <snehapatil02@git.wordpress.org>
Co-authored-by: Mamaduka <mamaduka@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
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[Mamaduka]

The 100 is the default limit of the REST API; changing it to 500 would result in an error. Additionally, we can't use unbound query here due to the performance issue noted above

[t-hamano]

Can we close this PR? Similar limitations exist elsewhere:

gutenberg/packages/block-library/src/comment-template/hooks.js

Line 11 in 973e916

const MAX_COMMENTS_PER_PAGE = 100;

gutenberg/packages/editor/src/components/post-taxonomies/flat-term-selector.js

Line 39 in 973e916

const MAX_TERMS_SUGGESTIONS = 100;

Perhaps we should discuss the REST API limitation itself.

[t-hamano]

Let's close this PR and look for an ideal alternative approach.

@snehapatil2001 Thank you for your efforts!