Rewrite tests as modern pytest with Pydantic support

- Convert old commented unittest tests to pytest
- Add comprehensive test coverage for process_request, select_important_assembly, and prepare_structured_data
- Use pytest fixtures for better test organization
- Test Pydantic model validation
- Remove obsolete test_should_succeed function
- Update README example to reference actual test

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: mattgodbolt

README.md

@@ -64,7 +64,7 @@ The service will be available at http://localhost:8000
 uv run pytest
 
 # Run specific test
-uv run pytest app/explain_test.py::test_should_succeed
+uv run pytest app/explain_test.py::TestProcessRequest::test_process_request_success
 ```
 
 ### Code Quality