Allow Embed Optimizer to work with Classic Editor and Classic block embeds

[sarthak-19]

Summary

Fixes #2226.

Embed Optimizer's optimizations (dns-prefetch, layout-shift prevention, and Optimization Detective-aware lazy loading) previously only applied to embeds added via the block editor's Embed block, since Embed_Optimizer_Tag_Visitor only recognizes the FIGURE.wp-block-embed > DIV.wp-block-embed__wrapper markup that block saves.

Embeds added via the Classic Editor (auto-embedded URLs, the [embed] shortcode) or the Classic block render as bare oEmbed HTML with no such wrapper, so they were invisible to the tag visitor and missed out on these optimizations.

Both cases resolve through WP_Embed::shortcode() and fire the same embed_oembed_html filter — including the bare URL saved inside the Embed block's own wrapper, which is resolved by WP_Embed::autoembed() before the block is even parsed (autoembed() runs at the_content priority 8, do_blocks() at priority 9). This PR hooks that filter to wrap the resolved HTML in the same FIGURE/DIV markup the Embed block uses, so classic embeds become recognizable to the tag visitor just like Embed-block ones.

To avoid double-wrapping the Embed block's own markup (since its bare URL is resolved through the exact same filter), wrapping is skipped when the URL is already found inside an existing wp-block-embed__wrapper in the post's stored content.

• plugins/embed-optimizer/helper.php: adds the wrapping filter, a helper to detect an existing Embed-block wrapper, and a best-effort URL-to-provider-class mapping (for the wp-block-embed-{provider} class used by dns-prefetch matching).
• plugins/embed-optimizer/tests/test-hooks.php: unit coverage for the new functions plus an integration test exercising the real WP_Embed class for both the classic and Embed-block cases.
• plugins/embed-optimizer/readme.txt: drops the "not inside a Classic Block" caveat.

Trade-off to note

Wrapping previously-unwrapped classic embed HTML in FIGURE.wp-block-embed pulls in the block library's default .wp-block-embed spacing/CSS for those embeds (most themes already enqueue block styles site-wide). This is a minor, expected visual change and is the intended fix, but worth a second look from reviewers. Widget content (widget_text_content/widget_block_content) is not covered by the double-wrap guard, since there's no post ID to check against there; this is a known limitation left for follow-up since the issue is specifically about post content.

Test plan

• composer phpstan — 0 errors
• composer lint -- --standard=plugins/embed-optimizer/phpcs.xml.dist — 0 errors
• plugins/embed-optimizer PHPUnit suite (86 tests) passing against a real wp-env WordPress install
• optimization-detective PHPUnit suite (359 tests) passing, confirming no regressions in the tag-visitor pipeline

[codecov]

Codecov Report

❌ Patch coverage is 97.67442% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 70.65%. Comparing base (1647b94) to head (08cd395).

Files with missing lines	Patch %	Lines
plugins/embed-optimizer/helper.php	97.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2685      +/-   ##
==========================================
+ Coverage   70.50%   70.65%   +0.14%     
==========================================
  Files          91       91              
  Lines        7869     7912      +43     
==========================================
+ Hits         5548     5590      +42     
- Misses       2321     2322       +1     

Flag	Coverage Δ
multisite	70.65% <97.67%> (+0.14%)	⬆️
single	35.57% <79.06%> (+0.23%)	⬆️

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