refactor: opinionated tweaks to pystapi-client tests

[gadomski]

What I'm changing

• Tweak some import styles to @gadomski's preference (classes imported directly)
• Rename mocked_api to api since it's unlikely we'll ever have a real API in the test suite
• Use http://stapi.test instead of https://stapi.example.com (@gadomski prefers .test to make it clear that urls are for a test case ... example.com actually resolves to something, wheras .test never will)

Checklist

• Tests pass: uv run pytest
• Checks pass: uv run pre-commit --all-files