Add tests for global DOM engine behavior

[thibaudcolas]

Motivation

• The DOM engine selection is stored on DOM.dom and HTML.__init__ calls DOM.use, causing the engine to be global across all HTML instances and leading to surprising last-wins and thread-safety issues.
• Add unit tests that capture the current (problematic) behavior so future fixes can be validated with regressions.

Description

• Added two unit tests to tests/test_html.py: test_engine_is_global_for_existing_instances and test_engine_switching_affects_previous_instance that exercise engine selection across multiple HTML instances.
• The tests assert the difference in escaping behavior between DOM.STRING and DOM.STRING_COMPAT to demonstrate that switching engines affects previously created HTML instances.

Testing

• No automated tests were executed as part of this change; the new tests were added but not run here.
• To run the tests locally, execute make test or pytest tests/test_html.py::TestHTML::test_engine_is_global_for_existing_instances tests/test_html.py::TestHTML::test_engine_switching_affects_previous_instance.

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Codex Task