fix: [#2229] Fix Element.setAttribute() to accept names containing a colon (e.g. hx-on:click)

[marciomazza]

Description

Resolves #2229

Document.createAttribute() (and, through it, Element.setAttribute()'s HTML-namespace path) was splitting the qualified name on : into prefix/localName, even though that's only correct for createAttributeNS()/setAttributeNS(). Per spec, plain createAttribute() must keep the qualified name whole, with prefix = null and no namespace.

This produced an Attr with a prefix but no namespaceURI — a combination NamedNodeMap#getNamedItemNS() correctly refuses to resolve, since real namespaced attributes always have both. That made setNamedItem() unable to find the previous attribute to replace, so it kept appending instead of overwriting: getAttribute() on any colon-containing, non-namespaced attribute name (e.g. hx-on:click, used by htmx 4, or any custom foo:bar-style attribute) would forever return the very first value ever set via setAttribute(), no matter how many times it was called afterward.

Fix: createAttribute() now always sets localName to the full qualified name and prefix to null, matching the DOM spec and real browser behavior. createAttributeNS()/setAttributeNS() are untouched — they already implement the namespace-aware split correctly.

Also tidied up three existing tests (Element.test.ts / NamedNodeMap.test.ts) that were titled around "same local name, different prefix" but exercised via plain setAttribute() — which never namespaces anything, so they weren't actually testing that case. Rewrote them to use setAttributeNS() with real distinct namespaces, which was previously uncovered.

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AI disclosure

Per the project's AI Contributions policy: Claude (Anthropic) assisted in diagnosing the bug, writing the fix, and drafting this description. I guided and reviewed the change myself at every step.