fix(tocObj): extract semantic math text

[stevenjoezhang]

check list

• Add test cases for the changes.
• Passed the CI test.

Description

Fixes duplicate or incorrect TOC text when headings contain math rendered by KaTeX or other MathML-based renderers.

KaTeX outputs both an accessible MathML representation and a visual HTML representation. DomUtils.textContent() concatenated both branches, causing formulas to appear multiple times in generated TOC entries.

This change adds semantic text extraction that:

• ignores elements marked with aria-hidden="true" or hidden
• prefers the TeX annotation inside MathML <semantics>
• falls back to the presentation MathML branch when no TeX annotation exists
• preserves the existing permalink filtering behavior

No additional math-rendering dependency is introduced.

Additional information

To test:

• npm test (260 passing)
• npm run eslint
• Hexo integration test with the patched local hexo-util on Node.js 24 (1278 passing, 5 pending)
• Verified a KaTeX heading such as Energy E=mc^2 appears only once in the generated TOC

Fixes #447