Media: Skip cross-origin isolation for third-party page builders

[adamsilverstein]

Summary

• Adds a check to skip Document-Isolation-Policy (DIP) when a third-party page builder overrides the block editor via a custom action query parameter
• DIP isolates the document into its own agent cluster, which blocks same-origin iframe access that page builders rely on
• Split out from Media: Use Document-Isolation-Policy for cross-origin isolation #11098 per review feedback to allow separate discussion of edge cases (e.g. Web Stories uses action=edit but replaces the block editor entirely)

See discussion: #11098 (comment)

Test plan

• Verify the block editor still gets cross-origin isolation headers on standard post/page edit screens
• Verify that a page builder using a custom action parameter (e.g. action=elementor) does NOT get DIP headers
• Discuss edge cases where action=edit is used but the block editor is replaced (e.g. Web Stories)

Trac ticket: https://core.trac.wordpress.org/ticket/64803

[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 adamsilverstein.

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.

[westonruter]

Which third party page build(s) specifically is known to have an issue that this fixes?