Fix automatic links split by HTML entities

[ryanchou1994]

Changes

• Buffer eligible automatic-link labels until the complete anchor text is available.
• Preserve regular-link output when the label differs, contains nested markup, or automatic links are disabled.
• Add regression coverage for HTML entities, streamed parser input, and escaping options.

Fixes #441

Tests

• uv run --with pytest pytest -q (200 passed)
• Linux python:3.13 container, pytest -q (200 passed)
• black --check --diff html2text/__init__.py test/test_html2text.py
• flake8 html2text/__init__.py test/test_html2text.py
• isort --check-only --diff html2text/__init__.py test/test_html2text.py
• mypy --strict html2text

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