Optimization Detective: Handle stray closing P tags when P is not open

[wprashed]

Fixes #2650

Description

When wpautop() wraps multiple shortcodes (e.g. <p><figure>...</figure><figure>...</figure></p>), encountering the first <figure> causes OD_HTML_Tag_Processor to implicitly pop P off the open tag stack (since FIGURE is in P_CLOSING_TAGS).

When the trailing </p> tag is later encountered, P is no longer at the top of ->open_stack_tags. Previously, OD_HTML_Tag_Processor called array_pop( $this->open_stack_tags ), popping a parent element (such as <div class="entry-content">) off the stack. This corrupted the open tag stack for the rest of the document, emitting warnings and producing incorrect data-od-xpath attributes on subsequent elements.

This PR updates OD_HTML_Tag_Processor to:

1. Detect when a </p> tag is encountered while P is not at the top of ->open_stack_tags.
2. Increment $this->open_stack_indices[ $level ] to match browser DOM behavior (where an implied empty <p> element occupies a sibling slot).
3. Return true early without popping parent tags or emitting false warnings.

Testing Instructions

1. Review plugins/optimization-detective/class-od-html-tag-processor.php and plugins/optimization-detective/tests/test-class-od-html-tag-processor.php.
2. Confirm that HTML containing <p><figure>...</figure><figure>...</figure></p> parses without warnings and computes accurate XPath breadcrumbs for subsequent elements.

[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.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: wprashed <wprashed@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[westonruter]

There is already an open PR for this: #2651

Please carefully review an issue and any associated PRs before starting your own.