test: Add coverage for :is()/:matches() containing nested :has() selectors

[Copilot]

The parser threw ParseError: Missing closing ')' when parsing pseudo-selectors like :is(:not(:has(>.foo)), :has(>.foo.bar)), despite the parentheses being balanced. The fix was already applied in PR #4290 (changing the element fallback parser to loop through comma-separated selector lists rather than expecting a single selector before )), but the specific scenario from this issue had no test coverage.

Changes

• tests-unit/selectors/selectors.less — Added test cases for:
  • :is(:not(:has(>.foo)), :has(>.foo.bar)) — the exact failing case
  • :matches(:not(:has(>.foo)), :has(>.foo.bar)) — :matches() variant
  • :is(:has(>.foo + .bar), :has(>.baz ~ .qux), :not(:has(.quux))) — multiple complex nested :has() arguments
• tests-unit/selectors/selectors.css — Corresponding expected output

// Previously threw: ParseError: Missing closing ')'
:is(:not(:has(>.foo)), :has(>.foo.bar)) {
  overflow: clip;
}

Original prompt

---

This section details on the original issue you should resolve

<issue_title>ParseError with :is() or :matches() containing :has()</issue_title>
<issue_description>To reproduce:

:is(:not(:has(>.foo)), :has(>.foo.bar)) {
    overflow: clip;
}

Current behavior:

Throw a:
ParseError: Missing closing ')'

Expected behavior:

No exception has the number of close parenthesis is correct
Environment information:

• less version: 4.2.0
• nodejs version: 20.17
• operating system: windows</issue_description>

<agent_instructions>Can we get proper selector parsing for known pseudo-selectors that can have selectors?</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@matthew-dean Related to: https://github.com//issues/4268 @puckowski I submitted a PR #4290

The following Less

@num: 3;
:nth-child(@{num}) {
  selector: interpolated;
}


a:is(.b, :is(.c)) {
  color: blue;
}

:is(:not(:has(>.foo)), :has(>.foo.bar)) {
  overflow: clip;
}

becomes:

:nth-child(3) {
  selector: interpolated;
}
a:is(.b, :is(.c)) {
  color: blue;
}
:is(:not(:has(>.foo)), :has(>.foo.bar)) {
  overflow: clip;
}
```</body></comment_new>
</comments>

• Fixes ParseError with :is() or :matches() containing :has() #4278

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