attr(), addClass(), hasAttr() and other attribute/class methods fatal on non-element nodes in the match set #98

Several attribute and class methods call element-only DOM methods (getAttribute(), setAttribute(), hasAttribute(), removeAttribute()) on every node in the match set without checking its type. When the set contains a text, comment, CDATA or processing-instruction node, as contents() returns, they fail with a fatal Error. tag() had the same bug (#93), but there it only produced a warning.

Reproduce

$q = qp('<?xml version="1.0"?><root>text<a class="c" x="1"/></root>', 'root')->contents();
$q->addClass('c');
// Error: Call to undefined method DOMText::hasAttribute()

Results on main (cd0de71, PHP 8.5) for two match sets. "Text first" is contents() of <root>text<a class="c" x="1"/></root>; "element first" is contents() of <root><a class="c" x="1"/>text</root>:

The setters, hasAttr() and removeAttr() loop over every match, so one non-element node anywhere in the set breaks them. The getters read only the first match, so they break only when a non-element node is first.

Suggested fix

Fix this once at the boundary rather than with a guard in each method, as commit 745a4e2 did for the CSS engine. For example:

• add a DOM helper that returns the first match that is a DOMElement, or null, plus a way to iterate only the element matches;
• route tag() and the methods above through it.

That would make non-element nodes behave like jQuery's: setters skip them, and getters act as if they weren't in the set. For the getters, decide whether "first match" means the first node or the first element. #93 kept tag() reporting the first node and returning '' for a non-element. The getters here could do the same, returning null or false, or skip ahead to the first element; whichever is chosen, it should be applied consistently.

Found while reviewing #97.