test(server): enforce coverage threshold

[hittyt]

Summary

• Enforce the server CI coverage gate with pytest-cov and --cov-fail-under=80.
• Upload the server coverage.xml report from the server test workflow for coverage evidence.
• Add focused server tests for Docker diagnostics, Kubernetes diagnostics, and secure runtime resolution so the full opensandbox_server package exceeds 80% statement coverage.

Testing

• Not run (explain why)
• Unit tests
• Integration tests
• e2e / manual verification

Commands run:

• cd server && uv run ruff check
• cd server && uv run pytest --cov=opensandbox_server --cov-report=term --cov-report=xml:reports/coverage.xml --cov-fail-under=80 (972 passed, total coverage 80.28%)

Breaking Changes

• None
• Yes (describe impact and migration path)

Checklist

• Linked Issue or clearly described motivation
• Added/updated docs (if needed)
• Added/updated tests (if needed)
• Security impact considered
• Backward compatibility considered