fix(Paragraph): addChild() should throw on invalid children instead of silent discard

[usernane]

Bug Description

Paragraph::addChild() silently discards elements not in ALLOWED_CHILDREN. No exception, no warning — the child vanishes.

$p = new Paragraph();
$p->addChild(new HTMLNode('div')); // Silently ignored
$p->childrenCount(); // 0

Proposed Fix

Throw InvalidArgumentException listing allowed elements:

public function addChild($node, $attrsOrChain = [], bool $chainOnParent = true): HTMLNode {
    $nodeName = $node instanceof HTMLNode ? $node->getNodeName() : $node;

    if (!in_array($nodeName, self::ALLOWED_CHILDREN) && $nodeName !== HTMLNode::TEXT_NODE) {
        throw new \InvalidArgumentException(
            "Element '$nodeName' is not allowed in <p>. Allowed: "
            . implode(', ', self::ALLOWED_CHILDREN)
        );
    }
    // ... proceed with add
}

Related

• fix(HeadNode): addChild() should throw on invalid children instead of silent discard #70 — Same fix pattern for HeadNode::addChild()

Acceptance Criteria

• Invalid children throw InvalidArgumentException with allowed list
• Text nodes still added without error
• All existing Paragraph tests updated