Fix duotone filter not applying on style variation switch

[prachigarg19]

What?

Closes #69847
Fix duotone filter not applying when switching style variations in the Site Editor for template parts.

Why?

When switching to a style variation that includes a duotone filter (e.g. the "Midnight" variation in Twenty Twenty-Five theme), the duotone effect was not applied to blocks inside template parts (such as the Site Logo in the header) until the page was manually refreshed.

The root cause was a type string mismatch in the global styles pipeline. In packages/global-styles-engine/src/core/render.ts, the SVG filter assets were tagged with __unstableType: 'svg', but EditorStyles in packages/block-editor/src/components/editor-styles/index.js filters for __unstableType === 'svgs' (with an s). Because of this mismatch, the globally generated duotone preset SVG filter definitions were silently dropped and never injected into the DOM, so the CSS filter: url(#wp-duotone-midnight-filter) rule had nothing to reference.

How?

Changed __unstableType: 'svg' to __unstableType: 'svgs' in generateGlobalStyles inside packages/global-styles-engine/src/core/render.ts to match the value that EditorStyles expects.
This is a one-character fix. The SVG filters were already being correctly generated for all style variations via generateSvgFilters (including non-active variations via getThemeVariationSettingsNodes), but were never consumed by EditorStyles due to this typo.
No other code in the codebase reads __unstableType === 'svg' — confirmed by grep. All other writers of __unstableType for SVGs already use 'svgs', so this change brings render.ts in line with the rest of the codebase.

Testing Instructions

1. Activate the Twenty Twenty-Five theme.
2. Open the Site Editor (Appearance → Editor).
3. Navigate to Header and edit it.
4. Add a Site Logo block to the header.
5. Save.
6. Open the Styles panel in Editor.
7. Click Browse styles and switch from Default to Midnight.
8. Observe that the duotone filter is now applied to the site logo immediately without requiring a page refresh.

Testing Instructions for Keyboard

This PR does not change any UI interactions. The fix is purely in the styles pipeline. No keyboard-specific testing is required.

Screenshots or screencast

Before	After
Duotone filter not visible after switching to Midnight variation until page refresh	Duotone filter applied immediately upon switching to Midnight variation

Use of AI Tools

None

[github-actions]

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Co-authored-by: prachigarg19 <prachigarg19@git.wordpress.org>
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[t-hamano]

Thanks for the PR!

My guess is this was a simple oversight in #49239, and this change makes sense to me. Just to be sure, I'll have @scruffian and @andrewserong confirm.

[andrewserong]

My guess is this was a simple oversight in #49239, and this change makes sense to me.

Yes, that looks like it to me, everywhere else it's svgs 👍

Nice catch @prachigarg19 and thanks for the fix!

LGTM 🚀

[carolinan]

Great find, thank you @prachigarg19