test: add helper function verification for RequirementsExtractor

- Add manual test for RequirementsExtractor utility functions
- Tests split_markdown_for_llm, parse_md_headings, merge functions
- Tests JSON extraction and validation helpers
- Provides executable verification of low-level utilities
- Complements existing integration tests in test/manual/
- Moved to test/manual/ for consistency with other manual tests

Author: vinod0m