Strengthen json-to-xml test assertions with DOM validation

[Copilot]

Addresses review feedback on PR #154 regarding weak test assertions in json-to-xml.test.tsx.

Changes

• Replace string inclusion checks with DOM-based assertions in the "should convert simple string JSON" test
  • Parse output XML into DOM structure
  • Verify root element and structure
  • Extract and validate actual text content from text nodes

Before/After

// Before: passes even if structure is wrong
assert(outXmlString.includes('hello'));

// After: validates actual DOM structure
const outXml = xmlParser.xmlParse(outXmlString);
assert.equal(outXml.documentElement.nodeName, 'result');
const child = outXml.documentElement.firstChild;
const textContent = extractTextContent(child);
assert.equal(textContent, 'hello');

The test now catches structural regressions that substring matching would miss (wrong element names, incorrect nesting, malformed output).

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