refactor: finalize Google Doc HTML parsing migration (PR 9128)

- Migrated from cheerio to node-html-parser for better footprint and performance.
- Implemented robust block-element spacing (div, p, h1-h6, li, etc.) using insertAdjacentHTML.
- Normalized whitespace to single spaces.
- Comprehensive unit tests (14 cases) achieving 100% coverage.
- Addressed all reviewer feedback (arii approved).
- Final cleanup of temporary exploration files.

Co-authored-by: arii <342438+arii@users.noreply.github.com>

Author: google-labs-jules[bot]