Requirements Library: Test coverage and test infrastructure setup

[akihikokuroda]

Set up overall test infrastructure for consistent testing across all sub-tasks.

Setup

• Review/update test/conftest.py for requirements-specific fixtures
• Create fixtures for temp directories, mock code samples, sandboxed environments
• Ensure test/stdlib/requirements/ mirrors source structure
• Verify all test files include appropriate pytest markers

Coverage

• Run coverage report: target >90% coverage on core requirements
• Document any intentional exclusions

Part of #1023