Fix stale steward submission tests

[luch91]

Summary

• add the missing multiplier fixture required by steward acceptance tests
• update steward stats assertions to match the current public stats endpoint behavior

Verification

• python3 manage.py test contributions.tests.test_steward_permissions -v 2
• verified in the repo's Python 3.13 Docker environment

[JoaquinBN]

Thanks @luch91 — the missing fixture and assertion corrections are valid. We'll address test maintenance internally as part of broader cleanup. We're going through a full review and rebranding right now, and once that's complete we'll open up clear issues for community contributions. Appreciate you flagging these!