feat: [#2393] support the :valid and :invalid CSS pseudo-classes

[marciomazza]

Description

Resolves #2393

:valid / :invalid are never matched — SelectorItem.matchPseudoItem has no
case for them, so any such selector silently returns nothing.

document.body.innerHTML = `<input id="a" required><input id="b" required value="x">`;
document.querySelectorAll(':invalid').length; // happy-dom: 0 — real browsers: 1

Fix: add an invalid / valid case. It is gated on willValidate so it only
matches constraint-validation candidates (skipping disabled / non-applicable
elements, as browsers do), then reflects checkValidity().

Deviation left on purpose: :valid / :invalid are not implemented for <form>
and <fieldset> themselves (they would need aggregate validity); only individual
form controls are covered.

Tests

Added 1 test: :invalid matches an empty required input, :valid matches a
filled one, and a disabled required input plus a <div> match neither.
Fails before / passes after. Full npm test passes.

AI

I used Claude Code to write this, over multiple iterations. I guided and reviewed
it myself at every step.

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Tests

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  • Ran the repro in Chrome 152: :invalid matched #a only, :valid matched #b only; the disabled required input and the <div> matched neither. Same as the test asserts.
• Run the tests with npm test locally to make sure that all tests pass before submitting the PR.

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