docs: add Python testing best practices guide

[agent-kurouto]

Summary

• Adds docs/testing.md — a comprehensive Python testing best practices guide covering unit tests, integration tests, E2E tests, property-based testing (Hypothesis), mocking & patching (pytest-mock), test organisation, and coverage
• Updates README.md to include the new guide in the contents table

Guide sections

1. Unit Tests — pytest basics, fixtures (with scope table), @pytest.mark.parametrize
2. Integration Tests — testcontainers, marking tests, selective test runs
3. End-to-End Tests — httpx async client, CLI subprocess testing
4. Property-Based Testing — Hypothesis @given, strategies, shrinking
5. Mocking & Patching — unittest.mock, pytest-mock, when not to mock
6. Test Organisation — conftest.py, naming conventions, pyproject.toml config
7. Coverage — pytest-cov, threshold enforcement, HTML reports, # pragma: no cover
8. Worked Example — complete mini-project tying all sections together

Test plan

• Review docs/testing.md for accuracy and consistency with existing guides
• Verify all code examples are syntactically correct
• Confirm README.md contents table renders correctly
• Check all internal links resolve

Closes PLT-1027

🤖 Generated with Claude Code

[eywalker]

Can we add a section that discusses what tests users should do if they are just getting started. Listing all different kinds of available tests can be overwhelming

[agent-kurouto]

Response to review comments

@eywalker — Can we add a section that discusses what tests users should do if they are just getting started. Listing all different kinds of available tests can be overwhelming

Done! Added a new Section 1 — "Where to Start" (commit 54cc0a8) with an explicit four-step on-ramp for beginners:

1. Step 1 — Install pytest and write a single test (the absolute minimum to get going)
2. Step 2 — Cover core business logic with unit tests, focusing on the scenarios that matter most
3. Step 3 — Add one or two smoke integration tests once unit tests are solid
4. Step 4 — Reach for E2E tests, property-based tests, mocking, and coverage enforcement only when a specific need arises, with a bullet-point guide on when that is:
   • Break a workflow → E2E test
   • Discover an unexpected edge-case bug → property-based test
   • Slow/flaky test from an external service → mocking
   • Prevent coverage regression in CI → coverage threshold

The existing sections are unchanged and now serve as the reference material that "Where to Start" links readers to when they're ready.

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PR summary

This PR adds docs/testing.md — a new standalone Python testing best practices guide for nauticalab/python-best-practices. It follows the same structure and style as the existing docs/logging.md.

What's covered

Section	Content
Where to Start	Beginner-friendly four-step on-ramp
Unit Tests	pytest fixtures, parametrize, scopes
Integration Tests	testcontainers, test markers, selective runs
End-to-End Tests	httpx async client, CLI subprocess testing
Property-Based Testing	Hypothesis @given, strategies, shrinking
Mocking & Patching	unittest.mock, pytest-mock, when not to mock
Test Organisation	conftest.py, naming conventions, pyproject.toml config
Coverage	pytest-cov, CI threshold, HTML reports, # pragma: no cover
Worked Example	Complete mini-project tying all sections together

README.md has also been updated to include the new guide in the contents table.