Address review: remove defaults in from_api_response, delete accidental design doc

- Use direct key access for required API fields (ReferencedEntity, etc.)
  instead of .get() with empty string defaults
- Keep .get() only for ReferencingEntityNavigationPropertyName which may
  be absent in the API response
- Remove docs/design/relationships_integration.md (old design doc
  accidentally included in reorg commit)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tpellissier