Fix: Duotone handle block style variations

[DarkMatter-999]

What?

Closes #77539

This PR ensures that duotone filters applied to block style variations in Global Styles (e.g., a "Rounded" variation of an Image block) are correctly rendered on the frontend and take precedence over the base block's duotone settings.

Why?

Previously, the WP_Duotone_Gutenberg class only checked for duotone settings applied to the base block type. If a user styled a specific variation with a different duotone, the frontend would either fallback to the base block's filter or apply no filter at all. This occurred because the logic did not account for the variations path in theme.json and didn't check the block's className for is-style-{variation} triggers during render.

How?

• Updated get_all_global_style_block_names to iterate through block variations in the theme.json data. It now stores variation-specific duotones using the key format blockName:variationName.
• Updated render_duotone_support to parse the block's className attribute. If an is-style- class is found that matches a stored variation, that duotone is prioritized.
• Added a check to ensure the style variation duotone is enqueued via enqueue_global_styles_preset before falling back to the default block duotone.
• Added PHPUnit tests to verify both the rendering of variation duotones and the correct precedence when both a base block and a variation have duotones defined.

Testing Instructions

• Using a block theme, go to Editor > Styles > Blocks > Image.
• Set a Duotone filter for the base Image block.
• In the same menu, select the "Rounded" variation and set a different Duotone filter.
• Save the changes.
• Create a post with two Image blocks: leave one as "Default" and set the other to the "Rounded" style.
• View the frontend.
• Expected: The default image and the rounded image should have different filters as we had set.

Screenshots or screencast

Before:

style-variation-duotone.mov

After:

style-variation-duotone-fix.mov

Use of AI Tools

I used Gemini 3 Flash to help draft the PR description and structure the summary based on my code changes. And Deepseek v4 Flash (via OpenCode) for initial PR review.

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Co-authored-by: DarkMatter-999 <lakshyajeet@git.wordpress.org>
Co-authored-by: adithya-naik <adithyanaik@git.wordpress.org>
Co-authored-by: dpmehta <mehtadev@git.wordpress.org>
Co-authored-by: tellthemachines <isabel_brison@git.wordpress.org>

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[DarkMatter-999]

I am happy to backport this to Core after the review 😇