fix: address review concerns - remove vitest mocking and update docs

- Fix resource handler to support dependency injection with optional executor parameter
- Rewrite resource tests to use only createMockExecutor() (no vitest mocking)
- Fix MIME type from ''application/json'' to ''text/plain'' to match content
- Completely rewrite docs/TESTING.md to align with CLAUDE.md testing standards
- Remove all banned vitest mocking patterns from documentation
- Add comprehensive dependency injection guidelines and troubleshooting

Addresses review feedback about test violations and outdated documentation.
All tests now follow the project''s strict no-vitest-mocking architecture.

Co-authored-by: Cameron Cooke <cameroncooke@users.noreply.github.com>

Author: claude[bot]