[BlockMarkupURLProcessor] Support URLs in CSS

[adamziel]

Description

Adds support for migrating URLs within CSS syntax in the style HTML attribute during WXR imports. For example, this markup:

<div style="background-image:url(https://oldsite.com/image.png)">

Would be rewritten as:

<div style="background-image:url(https://newsite.com/image.png)">

Motivation

When importing WordPress sites via WXR, URLs embedded in CSS (like background: url("/old-site.com/image.jpg")) need to be migrated to the new site. Previously, these URLs were missed, leading to broken images and assets after import.

Cover blocks are a good example. Without this PR, the background image in this cover block would not be rewritten:

<!-- wp:cover {"url":"http://localhost:8881/wp-content/image.jpg"}} -->
<div style="background-position:50% 50%;background-image:url(http://localhost:8881/wp-content/uploads/2025/09/image-2-766x1024.jpeg)">

Implementation

The implementation introduces a new CSSUrlProcessor class that can parse CSS url() functions, handle CSS escape sequences, and efficiently skip over large data URIs. It uses the same design principles as WP_HTML_Tag_Processor: simple state-machine API, no regexps, minimal allocations. The CSSUrlProcessor is integrated with BlockMarkupURLProcessor and can be used as follows:

$markup = '<div style="background: url(&quot;/old.jpg&quot;)">Content</div>';
$processor = new BlockMarkupUrlProcessor( $markup, 'https://new-site.com' );

while ( $processor->next_url() ) {
    // Finds "/old.jpg" in the style attribute
    $processor->set_raw_url( '/new.jpg' );
}

echo $processor->get_updated_html();
// Output: <div style="background: url(&quot;/new.jpg&quot;)">Content</div>

Testing instructions

• Review thoroughly
• Confirm the CI tests pass

[adamziel]

This seems to be looking good.

[adamziel]

@dmsnell would you mind taking a look at this one?

[dmsnell]

Thanks for the ping, @adamziel. I started doing review but it quickly turned into constant guessing which code was intentional and which was generated, making for a dizzying review experience. I hope I haven’t misjudged, but there are a number of fairly evident questions with the code.

It would help me to know how much effort you put into reviewing this so I can better asses the level of attention to give it.

It’s nice having the URLs inside of style attribute values remapped.

[adamziel]

Thank you @dmsnell! I've spent some time simplifying the initial LLM implementation, but it seems like I haven't gone deeply enough. You've asked some great questions that I should have caught earlier. Let me do another pass the slow, methodical way before asking you for another review.

[adamziel]

With the dedicated CSSProcessor class, this PR became much simpler. It's now a pretty natural extension of the BlockMarkupURLProcessor that merely adds another specialized handler for another subsyntax. Thank you @dmsnell for reviewing and giving me that extra push!