Strengthen test assertion for JSON property name sanitization

[Copilot]

The test for json-to-xml() property name sanitization used a weak substring check that could pass on false positives.

Changes:

• Parse XML output and verify element structure via DOM traversal
• Assert element name matches valid XML naming rules (/^[a-zA-Z_]/)
• Validate text content via text node's nodeValue
• Add node type assertions (element=1, text=3) for defensive programming

Before:

assert(outXmlString.includes('prop') || outXmlString.includes('_'));

After:

const propertyElement = rootElement.firstChild;
assert.strictEqual(propertyElement.nodeType, 1, 'Property should be an element node');
assert(/^[a-zA-Z_]/.test(propertyElement.nodeName), 'Element name should start with letter or underscore');
assert(propertyElement.nodeName.includes('prop'), 'Element name should contain prop');

const textNode = propertyElement.firstChild;
assert.strictEqual(textNode.nodeType, 3, 'First child should be a text node');
assert.strictEqual(textNode.nodeValue, 'value', 'Element should contain the correct value');

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