[codex] Fix centered HTML RTL compatibility

[omers-oai]

Codex: Fix centered HTML rendering in RTL compatibility mode.

What changed

• render AstHtmlBlock directly as block content instead of wrapping it in inline placeholder content
• add Roborazzi preview repro coverage for centered neutral and Hebrew HTML with RTL compatibility both on and off

Why

• the document-level intrinsic-width wrapper used by RTL compatibility interacted badly with inline HTML block placeholders
• centered HTML then collapsed into a vertical stack instead of staying horizontally laid out

Impact

• centered HTML remains readable and horizontal when enableRtlCompatibility = true
• the existing RTL document-width behavior stays covered by the preview suite

Testing

• ./gradlew :android-sample:testDebugUnitTest --console=plain
• ./gradlew :android-sample:recordRoborazziDebug --console=plain
• ./gradlew :richtext-markdown:allTests --console=plain
• ./gradlew :android-sample:testDebugUnitTest :richtext-markdown:allTests --console=plain

[tananaev]

Is there a reason we don't just always do this?

[omers-oai]

Unclear if this actually fixes something, but the goal here was to minimize the changes to an experiment so we if some weird case breaks it we have control to back it up.

I'm going to close this, since Codex wrongly thought another PR broke something, but it did not and this is not needed.