Editor GPU: Fix ghost text color

[Tyriar]

The problem was nested rules and multiple classes weren't supported properly.

Fixes #238859

[Copilot]

Pull request overview

This PR fixes ghost text color rendering in the GPU-accelerated editor by improving the CSS rule extraction logic to properly handle nested CSS rules and multiple space-separated class names.

Changes:

• Added support for extracting CSS rules when className contains multiple space-separated classes
• Implemented recursive extraction of nested CSS rules (CSS nesting support)
• Updated the regex pattern to recognize chained class selectors (e.g., .class1.class2)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/vs/editor/browser/gpu/css/decorationCssRuleExtractor.ts	Refactored rule extraction into a recursive _collectMatchingRules method; added support for space-separated class names and nested rules; updated regex to include . for chained selectors
src/vs/editor/test/browser/gpu/decorationCssRulerExtractor.test.ts	Added tests for pseudo-class selectors and multiple space-separated class names

Comments suppressed due to low confidence (1)

src/vs/editor/test/browser/gpu/decorationCssRulerExtractor.test.ts:115

• The PR description mentions fixing support for "nested rules" but there's no test case covering CSS nesting (e.g., .parent { .child { color: red; } }). Consider adding a test to verify that nested CSS rules are properly extracted, especially since lines 94-97 in the implementation specifically handle this case.

	});