Restore libxml error state after conversion

[OmeBlues]

Summary

• Restore previous libxml internal error state after HTML conversion
• Add tests to ensure state is restored and unchanged when suppression is disabled

Testing

• ./vendor/bin/phpunit (PHP 7.4.33)
• ./vendor/bin/phpunit (PHP 8.0.30)
• PHP >= 8.1: local failures in existing tests (XML header output; unrelated to this change)

Fixes #268

[Copilot]

Pull request overview

This PR fixes issue #268 by ensuring that libxml's internal error state is properly restored after HTML-to-Markdown conversion, preventing global state pollution that could affect other code using libxml.

Changes:

• Captured and restored the previous libxml error state after HTML conversion
• Added test coverage to verify state restoration when error suppression is enabled
• Added test coverage to verify state remains unchanged when error suppression is disabled

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/HtmlConverter.php	Modified createDOMDocument() to capture initial libxml error state and restore it in a finally block
tests/HtmlConverterTest.php	Added two test methods to verify libxml error state handling with and without error suppression

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