fix: reject self-closing tags as multiple root nodes in validator

[chatman-media]

Problem

XMLValidator.validate only detects multiple root elements when the roots contain child content. A self-closing root element silently passes validation, even though the XML 1.0 spec requires exactly one root element:

There is exactly one element, called the root, or document element, no part of which appears in the content of any other element.

const { XMLValidator } = require("fast-xml-parser");

XMLValidator.validate("<a></a><b></b>"); // correctly INVALID: "Multiple possible root nodes found."
XMLValidator.validate("<a/><b/>");        // BUG: returns true (valid)
XMLValidator.validate("<a/><b/><c/>");    // BUG: returns true (valid)
XMLValidator.validate("<a/>extra");       // BUG: returns true (valid)
XMLValidator.validate("<a></a><b/>");     // BUG: returns true (valid)

This is the self-closing counterpart of #207 / #210, which only fixed the paired-tag case.

Root cause

In src/validator.js, the reachedRoot flag is set to true only in the closing-tag branch when the tag stack empties. The duplicate-root guard (Multiple possible root nodes found.) and the trailing-text guard (Extra text at the end) both depend on that flag.

A self-closing tag is handled in its own branch and never sets reachedRoot, so a self-closing root never marks the document as complete. Any node that follows it therefore escapes both guards.

Fix

In the self-closing branch, when the tag is at depth 0 (tags.length === 0) it is a complete root element. Mark reachedRoot = true, and if the root level had already been reached, reject the second root with the existing Multiple possible root nodes found. error. This reuses the existing error codes and lets the trailing-text guard fire too.

Tests

Added to spec/validator_spec.js under the existing should not validate XML documents with multiple root nodes block. The new cases fail without the fix and pass with it; the full suite (319 specs) stays green, and single self-closing roots (<a/>, <a/> followed by whitespace/comments/PIs) remain valid.

• self-closing root followed by another self-closing node
• self-closing root followed by a paired node
• paired root followed by a self-closing node
• self-closing root followed by trailing text