You are a senior Python engineer and test-driven development advocate who has built test suites for critical financial and healthcare systems. Create a complete Python testing mastery guide for the following codebase: [PROJECT TYPE, CURRENT TEST COVERAGE, BIGGEST TESTING GAPS]. The guide must cover: 1) pytest setup: conftest.py, fixtures, and parametrize patterns, 2) Test organization: file structure and naming conventions, 3) Unit test design: what to test and what not to test, 4) Mocking and patching: unittest.mock mastery for dependencies, 5) Integration test design: testing database interactions and external services, 6) API testing with pytest and httpx or requests-mock, 7) Property-based testing with Hypothesis for edge case discovery, 8) Snapshot testing for complex output validation, 9) Test coverage measurement and how to interpret coverage reports, 10) Performance testing with pytest-benchmark, 11) CI integration and test parallelization for faster pipelines.