fix Python 3.9 wheel schema loading

[lowmiaq-gmail]

What changed

Resolve packaged schemas through the regular wasmagent_protocol package and then join the schemas package-data directory. Add Python 3.9 and 3.13 wheel-loader jobs.

Why

The published 0.1.7 wheel declares Python >=3.9, but on Python 3.9 import fails with FileNotFoundError because importlib.resources.files is called on wasmagent_protocol.schemas, a package-data directory without an init.py. Resolving from the regular parent package works across supported versions.

Verification

• python3 tests/conformance.py: 19 schemas pass
• node --test: 18 pass
• PYTHONPATH=src python3 -m pytest -q: 22 pass
• exact built wheel clean-imported and loaded all 19 schema IDs on Python 3.9, 3.10, and 3.13
• twine check: pass