Make Block Inserter search input sticky while scrolling

[sarthaknagoshe2002]

What?

Closes #71995

Makes the search input inside the block inserter panel sticky so it remains visible while scrolling through blocks.

Why?

Currently, the search bar scrolls out of view when browsing long lists of blocks. This change prevents users from having to unnecessarily scroll back to the top just to access the search functionality, improving overall workflow and usability.

How?

Updated the .block-editor-inserter__search SCSS to use position: sticky and top: 0. Added a z-index to ensure the scrolled blocks slide cleanly underneath the search input. Intentionally omitted the background color to allow the blocks to remain visible as they slide past the search area, creating a nice visual effect.

Testing Instructions

1. Open a post or page in the editor.
2. Click the + (Toggle block inserter) button in the top-left toolbar to open the inserter panel.
3. Scroll down through the list of available blocks.
4. Verify that the search input remains fixed at the top of the panel and that blocks scroll cleanly underneath it.

Screenshots or screencast

Before	After

[github-actions]

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Co-authored-by: sarthaknagoshe2002 <sarthaknagoshe2002@git.wordpress.org>
Co-authored-by: vincent-bdrt <vincentbdrt@git.wordpress.org>

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