Fix playlist block styling

[shimotmk]

What?

Related #43465

Why?

Adding box-sizing: border-box; to increase consistency with other blocks.

How?

Testing Instructions

1. Open a post or page.
2. Insert a playlist block.
3. Setting padding does not change the width of the block.

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

Use of AI Tools

[github-actions]

Size Change: +22 B (0%)

Total Size: 7.72 MB

📦 View Changed

Filename	Size	Change
build/styles/block-library/playlist/style-rtl.css	2.16 kB	+5 B (+0.23%)
build/styles/block-library/playlist/style-rtl.min.css	2.07 kB	+3 B (+0.15%)
build/styles/block-library/playlist/style.css	2.16 kB	+5 B (+0.23%)
build/styles/block-library/playlist/style.min.css	2.07 kB	+5 B (+0.24%)
build/styles/block-library/style-rtl.css	22.3 kB	+2 B (+0.01%)
build/styles/block-library/style.css	22.4 kB	+2 B (+0.01%)

_{compressed-size-action}

[noruzzamans]

Tested this on trunk, padding wasn't working correctly on the Playlist block (it changed the block's width instead of just adding padding), on this branch it works properly now.

Screen.Recording.2026-07-15.at.6.02.50.PM.mov

[github-actions]

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Co-authored-by: shimotmk <shimotomoki@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: noruzzamans <noruzzaman@git.wordpress.org>

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[github-actions]

Flaky tests detected in a75229b.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/29412110511
📝 Reported issues:

• [Flaky Test] Should navigate inner blocks with arrow keys #39607 in /test/e2e/specs/editor/various/writing-flow.spec.js

[t-hamano]

Nice catch!

[github-actions]

I just cherry-picked this PR to the wp/7.1 branch to get it included in the next release: 77073a7

[t-hamano]

I just cherry-picked this PR to the release/23.6 branch to get it included in the next release: 74593e0