Fix error when hovering over invalid :nth-child() selector

[Copilot]

Hovering an invalid :nth-child() selector in a CSS model threw out of the worker and reached the editor's error handling:

TypeError: Cannot read properties of undefined (reading 'getText')
    at ... selectorToSpecificityMarkedString
    at ... doHover

Root cause is in vscode-css-languageservice specificity calculation, which reads childElements[1].getText() for :nth-child(...) without checking that a second child exists — still present in 6.3.10 and upstream main, so it can't be resolved by a version bump.

Changes

• src/languages/features/css/cssWorker.ts: doHover catches errors from the language service and returns null, so an unparseable selector yields no hover instead of an unhandled rejection.

try {
    const hover = this._languageService.doHover(document, position, stylesheet);
    return Promise.resolve(hover);
} catch (e) {
    // The CSS language service can throw on invalid selectors, e.g. `:nth-child()`.
    return null;
}

• src/languages/features/css/cssWorker.test.ts: regression test covering :nth-child() (no hover, no throw) and li:nth-child(2n of .important) (hover still produced).
• package.json: test glob widened from src/languages/definitions/*/*.test.ts to src/languages/**/*.test.ts so feature-level tests run.

Notes for reviewers

The catch is deliberately scoped to hover only; other worker entry points are untouched. If the upstream parser bug is fixed and released, this guard becomes redundant but harmless.

• Fixes [Bug] Error when hovering over invalid :nth-child instruction #5430