Add lang attribute to the oEmbed iframe page HTML element

Gwenaelle Weberrequested to merge
issue/drupal-3593533:3593533-oembed-iframe-page into main

The media_oembed_iframe template renders a bare HTML page loaded inside a sandboxed iframe. Without a lang attribute on <html>, assistive technologies cannot determine the language of the embedded content, which violates WCAG 3.1.1 (Language of Page) and WCAG 3.1.2 (Language of Parts).

This adds a langcode variable (populated from the current content language in a preprocess hook) and uses it as lang="{{ langcode }}" on the <html> element.

Modules and themes that need provider-specific behaviour (e.g. adding hl= to a YouTube embed URL) can continue to do so via hook_preprocess_media_oembed_iframe().

Fixes #3593533

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