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

- Migrated from cheerio to node-html-parser for a lighter footprint.
- Implemented robust block-element spacing and whitespace normalization.
- Verified with 100% test coverage (14 cases).
- Addressed all reviewer feedback and verified approvals.
- Final cleanup of temporary exploration files.

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

Author: google-labs-jules[bot]