Twenty Twenty-One: Restore search block border in editor for Button inside style

[irozum]

In the Twenty Twenty-One theme, the Search block's "Button inside" style shows a border around the input/button group on the front end but not in the block editor. The theme's editor styles (_editor.scss) explicitly set border: none on the input for this style, originally relying on the block library's own front-end CSS to draw the border there. A later Gutenberg change (GB41393) lowered the specificity of that block-library rule, so the theme's own border rule now wins on the front end while the editor keeps hiding it — the two views have been out of sync since WordPress 6.1.

Per the ticket discussion, the maintainer team decided the editor should be updated to match the front end (the border has shipped on the front end since 6.1, so removing it now would be more disruptive than keeping it). This PR removes the border: none override in _editor.scss and regenerates the corresponding rule in the compiled style-editor.css, so the editor preview matches the front-end appearance.

Trac ticket: https://core.trac.wordpress.org/ticket/63888

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: Implementation and PR description, based on ticket discussion and root-cause analysis in the Trac comments. Verified against a local build with a live before/after comparison in the block editor. Reviewed by Igor Rozum.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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