fix: validator accepts multiple root nodes when a root is self closing

[yu2971512385-ui]

Description

XMLValidator already rejects documents with more than one root node, but the check is skipped whenever one of the roots is self closing:

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

XMLValidator.validate('<xml></xml><xml2></xml2>'); // error: Multiple possible root nodes found.  ✅
XMLValidator.validate('<xml/><xml2/>');            // true  ❌
XMLValidator.validate('<xml></xml><xml2/>');       // true  ❌
XMLValidator.validate('<test><nested/></test><xml2/>'); // true  ❌
XMLValidator.validate('<xml/>extra');              // true  ❌  ('<xml></xml>extra' is rejected)

All of these are not well-formed XML — a document has exactly one root element — and the validator's own error messages (Multiple possible root nodes found., Extra text at the end) exist for precisely these cases.

Root cause

In src/validator.js:

• reachedRoot = true is only assigned in the closing-tag branch, when popping a tag empties the stack. A self closing tag never reaches that branch, so it never ends the root level — hence '<xml/>extra' and '<xml/><xml2/>' pass.
• The Multiple possible root nodes found. check lives only in the branch for a paired opening tag, so a self closing tag after a completed root is never checked — hence '<xml></xml><xml2/>' passes.

Fix

In the self closing branch, once the attributes validate, treat a tag with an empty stack as a root node: report the multiple-root error if a root was already seen, otherwise mark the root level as reached. Nested self closing tags (<test><nested/></test>) are unaffected because the stack is not empty there.

Tests

Added to spec/validator_spec.js:

• multiple roots with the first root self closing, the second root self closing, and a self closing root after a root with nested tags
• a single self closing root (with and without attributes) and nested self closing tags stay valid
• text after a self closing root is rejected, trailing whitespace is still allowed

4 of the new specs fail without the source change. The full suite passes with it (npx jasmine spec/*spec.js → 338 specs, 0 failures, 2 pending).