[Py SDK] Add a test coverage check to the standard checks #1123

Summary

lib/python-sdk/Makefile has a test target but no coverage target, and the checks target does not run one. #493 standardized a Test (coverage) check across every package in the repo and closed with all three ACs checked, so the Python SDK is an outstanding gap against a standard we already called done.

Acceptance criteria

• Makefile has a coverage target that fails below an agreed threshold
• checks runs it
• CI invokes it the same way the TypeScript packages do