fix: validator; reject an XML declaration inside an element

[Shaisolaris]

Purpose / Goal

XMLValidator.validate reports "XML declaration allowed only at the start of the document." for a declaration before the root, but accepts one inside an element:

Input:

XMLValidator.validate('<foo><?xml version="1.0"?></foo>')
XMLValidator.validate('<foo>bar<?xml?></foo>')

Expected: InvalidXml error, "XML declaration allowed only at the start of the document."
Actual: true for both. libxml2 rejects both documents.

Cause: inside tag content the validator calls readPI(xmlData, ++i), which passes the index of the ? instead of the index after <?. readPI then reads the target as ?xml, so the tagname === 'xml' check never matches. The top-level call already passes the position after <?; this change does the same in tag content.

Type

• Bug Fix
• Refactoring / Technology upgrade
• New Feature

Testing:

• The new spec in spec/validator_spec.js fails before the change and passes after.
• npx jasmine "spec/*spec.js": 333 specs, 0 failures, 2 pending.
• The benchmarks in benchmark/ exercise the parser and builder, not the validator, so I did not include perf numbers. The change only alters an index argument.
• lib/ and the changelog are left for the maintainers, per CONTRIBUTING.