wp_kses: add support for picture element and srcset attribute on img tags

[adamsilverstein]

Trac ticket: https://core.trac.wordpress.org/ticket/29807

---

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.

[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

• The Plugin and Theme Directories cannot be accessed within Playground.
• 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.

[Copilot]

Pull Request Overview

This PR adds support for the HTML5 <picture> element and srcset attribute on <img> tags to WordPress's KSES filtering system. This enables proper handling of responsive images in content filtering.

• Adds <picture> and <source> elements to allowed post tags
• Adds srcset and sizes attributes to <img> and <source> elements
• Refactors URI sanitization logic to handle multi-URI attributes like srcset

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/wp-includes/kses.php	Adds picture/source elements to allowed tags, adds srcset/sizes attributes, and creates new wp_kses_sanitize_uris function to handle multi-URI attributes
tests/phpunit/tests/kses.php	Adds comprehensive test coverage for img srcset attributes, srcset sanitization, and picture element filtering

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}