Improve role fixtures (seed) to prevent missing RBAC data

Improve the role fixture (seed) mechanism so that RBAC-related data is never missing from seeded environments, and so that any missing data is easy to detect and verify.

JIRA Issue: BA-7113

Start by locating the role fixture and seed mechanism, then trace how RBAC-related data is created in seeded environments. Verify which RBAC data can be missing and define checks that make omissions easy to detect and verify; done means seeded environments consistently contain the required RBAC data and missing data is reported clearly.

Written by the indexing model from the issue text.