Fix: [Admin reskin] Add Plugins search field border is not OK on small screen

[hbhalodia]

Trac ticket: https://core.trac.wordpress.org/ticket/64809

• Target specific search form only when we have search results for the screen size.
• Else we are already having a margin of 11px.
• If we do not target specific when we have search results, it leads to extra spacing when there are no search results. So it goes to single line on specifc screen size without search results.

Use of AI Tools

• None

Screenshsots

1. With fix and search results

3. With fix and search results above 1250px.

--- **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](https://make.wordpress.org/core/handbook/contribute/git/github-pull-requests-for-code-review/) in the Core Handbook for more details.**

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props hbhalodia, huzaifaalmesbah, shailu25.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[huzaifaalmesbah]

I tested this PR fix at a 1140px screen width.
Before:

After:

[shail-mehta]

Plugin Search Border Issue is Resolved with PR✅

Before	After

[joedolson]

Why 1250px? WordPress doesn't use this breakpoint anywhere else, so I'm inclined to change this to 1200px unless there's a strong reason to leave it at 1250.