Boost: keep position-dependent inline scripts (document.write) in place when Defer JS is enabled

[kraftbj]

Fixes #30012

Proposed changes

• With "Defer Non-Essential JavaScript" enabled, inline scripts using document.write() / document.writeln() (e.g. added via a Custom HTML block) were relocated to after the footer, so their output rendered at the bottom of the page instead of where the block was placed.
• During output filtering, inline <script> tags (no src) whose body contains document.write are now tagged with the existing data-jetpack-boost="ignore" attribute, so they execute in their original position. The check is an intentionally conservative case-insensitive substring match — a false positive merely leaves a script in place, which is the safe outcome.
• No behavior change for: external scripts (src present, even if the inert body mentions document.write), plain inline scripts (still deferred), or scripts already carrying the ignore attribute (markup untouched, no duplicate attribute).
• Adds 6 unit tests exercising the module's real handle_output_stream() + append_script_tags() pipeline.

Related product discussion/links

• Long-standing customer report: [Boost] HTML Block with JavaScript Loads After Footer when Defer JS Enabled #30012
• Part of a Boost P0 reliability pass (audit follow-up); sibling PRs fix Page Cache uninstall cleanup (Fix cache garbage collection #41982/Boost: if there are too many files in the boost-cache directory, uninstall "fails" #41983) and Critical CSS SVG stripping (Boost: Inline SVGs stripped out of Critical CSS #42321).

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. On a test site, activate Jetpack Boost and enable Defer Non-Essential JavaScript.
2. Edit a post and add a Custom HTML block in the middle of the content:

   <div style="border:2px solid red">
   <script>document.write("<strong>WRITTEN BY SCRIPT</strong>");</script>
   </div>

3. Without this fix: view the post logged out — "WRITTEN BY SCRIPT" appears at the very bottom of the page after the footer; the red box is empty.
4. With this fix: the text renders inside the red box where the block was placed; view source confirms the script tag stayed in the content and carries data-jetpack-boost="ignore".
5. Confirm regular scripts are still deferred: view source and check theme/plugin scripts are still relocated to just before </body>.
6. Unit tests: composer install --working-dir=projects/plugins/boost && cd projects/plugins/boost && vendor/bin/phpunit-select-config phpunit.#.xml.dist --bootstrap tests/bootstrap.php --testsuite unit --filter Render_Blocking_JS_Test (22 tests, 28 assertions passing; full Boost unit suite: 120 tests, 340 assertions).

https://claude.ai/code/session_01PgpTrtTCH4hpz6Krh3ssho

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/plugins/boost/app/modules/optimizations/render-blocking-js/class-render-blocking-js.php	50/102 (49.02%)	38.79%	-27 💚

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