Toolbar: Constrain site icon to 20x20 regardless of page-level CSS overrides

[i-am-chitti]

Ticket

Trac ticket: https://core.trac.wordpress.org/ticket/65944

Description

The site icon in admin-bar.css is sized only via width/height, so a theme/plugin img rule using !important overrides it and the icon renders at intrinsic size instead of 20x20 (28x28 on mobile).

Adds max-width/max-height alongside the existing rules so the size holds even when width/height are overridden.

Testing

Set a Site Icon, then add img { width: auto !important; height: auto !important; } via Additional CSS — the icon should stay 20x20 with this fix (blows up without it).

Screenshots

Before	After

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Sonnet 5
Used for: reproducing the bug locally.

[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 iamchitti.

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.