fix: escape HTML-like tags in plain Markdown text

[lylelllll]

Motivation

Plain extracted text such as -a <address> currently passes through unchanged. Markdown renderers interpret <address> as raw HTML, so the placeholder is hidden or removed from rendered output.

Fixes #5360.

Modification

• Treat an unescaped < as a conservative Markdown special character, producing \< in plain-text output.
• Preserve the existing odd/even backslash behavior so already escaped placeholders are not double-escaped.
• Add focused regression tests for repeated placeholders, existing escapes, even backslashes, and the existing conservative special-character set.
• Leave code blocks and intentional table/algorithm HTML on their existing rendering paths.

BC-breaking

No public API changes. The generated Markdown source gains a backslash before unescaped < characters in plain text; rendered text remains the same literal content.

Tests

• PYTHONPATH=. uv run --python 3.12 --with pytest --no-project pytest -q -o addopts='' tests/unittest/test_markdown_utils.py (4 passed)
• uvx ruff check --ignore ANN001 mineru/backend/utils/markdown_utils.py tests/unittest/test_markdown_utils.py (passed; ANN001 is an existing annotation warning in the touched source file)
• CommonMark rendering smoke test confirmed \<address> renders as literal <address>, not an HTML element.
• Full model/GPU end-to-end parsing was not run because this change is isolated to the pure Markdown escaping utility.

Checklist

Before PR:

• Ruff was run on the modified files.
• The reported bug is covered by focused unit tests.
• Existing conservative escaping behavior is covered by a regression test.
• No documentation change is required for this narrowly scoped bug fix.

After PR:

• The behavior was verified with a CommonMark renderer.
• CLA has been signed and all committers have signed the CLA in this PR.

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[lylelllll]

I have read the CLA Document and I hereby sign the CLA