docs: clarify dev test setup

[randomsnowflake]

Summary

• Document the exact dev setup needed before running tests: install the package with .[dev].
• Explain why plain python -m pytest fails without the dev extra (requests-mock is test-only).
• Add a CI workflow that installs .[dev] and runs the offline test suite on Python 3.10-3.13.

Verification

• python -m pip install -e '.[dev]'
• python -m pytest -q — 49 passed
• python -m compileall -q src tests

Notes

• No runtime code changes.
• The workflow uses current GitHub Actions major versions (actions/checkout@v6, actions/setup-python@v6).