Clear CSS cache on theme change to prevent stale CSS variable values

[Copilot]

The CSS variable cache in DecorationCssRuleExtractor persisted across theme changes, returning stale values since CSS variables resolve differently per theme.

Changes

• Added clear() method to DecorationCssRuleExtractor: Invalidates both _ruleCache and _cssVariableCache
• Subscribe to theme changes in ViewGpuContext: Calls decorationCssRuleExtractor.clear() on onDidColorThemeChange, following the pattern established by TextureAtlas

// In ViewGpuContext constructor
this._register(this._themeService.onDidColorThemeChange(() => {
	ViewGpuContext.decorationCssRuleExtractor.clear();
	ViewGpuContext.atlas?.clear();
}));

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[Copilot]

Pull request overview

This PR fixes a bug where CSS variable values were cached across theme changes in the GPU-accelerated editor rendering, causing stale decoration styles to be used when users switched themes.

Key Changes

• Added a clear() method to DecorationCssRuleExtractor that invalidates both the CSS rule cache and CSS variable cache
• Subscribed to theme changes in ViewGpuContext to clear both the decoration CSS caches and texture atlas when the theme changes

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/vs/editor/browser/gpu/css/decorationCssRuleExtractor.ts	Added clear() method to invalidate cached CSS rules and CSS variable values
src/vs/editor/browser/gpu/viewGpuContext.ts	Injected IThemeService and subscribed to onDidColorThemeChange to clear decoration CSS caches and atlas