test(plugin-history-sync): restore fallbackActivity single-call and relay loadRef context tests

Restore the two tests removed in 2c1253c at maintainer request: fallbackActivity single-call per plugin instance and relay loadRef activity context loading. Also restore the relay fixture/dependency support needed by the context test. Verified that both restored tests pass against the reconciliation engine from 380ed94 and 56a7872.

Author: ENvironmentSet