HTML API: Fix the position update after changing the modifiable text when length differs

[gziolo]

Reported by @zieladam on GitHub: ​https://github.com/WordPress/wordpress-develop/pull/7007#issuecomment-2412540433

I just isolated the problem:

$p = new WP_HTML_Tag_Processor('Hello there');
$p->next_token();
$p->set_modifiable_text('Short');
echo $p->get_updated_html();

It's not occurring with this patch:

-               $this->bytes_already_parsed = $before_current_tag;
+               if ($this->get_token_type() === '#tag') {
+                       $this->bytes_already_parsed = $before_current_tag;
+               }

The actual fix will be more nuanced because the text node might appear in the HTML in the middle or at the end of the HTML provided, examples:

<p>Hello</p>String to replace

<p>Hello</p>String to replace<p>Hello</p>

Reported by @adamziel in ​https://github.com/WordPress/wordpress-develop/pull/7007#issuecomment-2412540433:

I just isolated the problem:

$p = new WP_HTML_Tag_Processor('Hello there');
$p->next_token();
$p->set_modifiable_text('Short');
echo $p->get_updated_html();

It's not occurring with this patch:

-               $this->bytes_already_parsed = $before_current_tag;
+               if ($this->get_token_type() === '#tag') {
+                       $this->bytes_already_parsed = $before_current_tag;
+               }

Trac ticket: https://core.trac.wordpress.org/ticket/62241

@gziolo I've proposed a fix for this in ​https://github.com/gziolo/wordpress-develop/pull/2 that can be merged into this PR.

@sirreal, I would appreciate another look after changes applied to unit tests.

[gziolo]

In 59250:

HTML API: Fix the position update after changing the modifiable text when length differs

There was an edge case detected for updating the text placed without the wrapping HTML tag with set_modifiable_text.

Props gziolo, jonsurrell.
Fixes #62241.

Committed with https://core.trac.wordpress.org/changeset/59250.

Thank you so much for a timely fix!