Fix infinite loop when using @variant inside @custom-variant that points to another @custom-variant

[RobinMalfait]

This PR fixes an infinite loop when you use a @variant inside of a @custom-variant, where the @variant used is another @custom-variant.

The issue stems from the fact that a @custom-variant can use a @slot that we have to replace with the proper AST nodes. However in this setup, the AST nodes will include a @slot node as well, which causes us to replace the @slot again, and so on, causing an infinite loop.

@custom-variant a {
  @slot;
}

@custom-variant b {
  @variant a {
    @slot;
  }
}

The solution here is to replace the @slot nodes and then skip walking the nodes that were just inserted. This does mean that we end up with a @slot node in the final AST but that's not a real issue because that will get replaced later when handling the next @custom-variant.

Test plan

1. Existing tests still pass
2. Added a regression test to ensure that the infinite loop does not happen anymore
3. Added additional tests to ensure that the behavior is correct

Thanks @wongjn for your initial debugging help and providing a test case as well!

Fixes: #19618

[coderabbitai]

Walkthrough

This pull request addresses an infinite loop issue occurring when using @variant inside @custom-variant. The changes include documenting the bug in the changelog, adding three new test cases to validate complex interactions between @custom-variant and @variant directives including nested variants and slot handling, and modifying the variant walking logic in variants.ts to use WalkAction.ReplaceSkip(nodes) instead of WalkAction.Replace(nodes) when handling @slot during AST traversal.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title clearly and specifically describes the main fix: preventing an infinite loop when using @variant inside @custom-variant that points to another @custom-variant, which matches the core change in variants.ts.
Description check	✅ Passed	The description is directly related to the changeset, explaining the infinite loop issue, the root cause involving @slot replacement, the solution implemented, and the test plan.
Linked Issues check	✅ Passed	The pull request successfully addresses the primary objective from issue #19618: fixing the infinite loop/crash when using @variant inside @custom-variant by changing @slot handling from Replace to ReplaceSkip in variants.ts, with comprehensive test coverage added.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the infinite loop issue: CHANGELOG.md documents the fix, variants.ts implements the core fix, and index.test.ts adds comprehensive test coverage for the specific issue and related scenarios.

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