SERVER-127013 (#56133) added replica set config validation requiring `electionTimeoutMillis` to be strictly greater than `heartbeatIntervalMillis`. The Antithesis mode of the resmoke replica set fixture (`NOOP_MONGO_D_S_PROCESSES`) sets `electionTimeoutMillis=2000`, which is equal to the server's default `heartbeatIntervalMillis` of 2000, so `replSetInitiate` now fails with:

```
InvalidReplicaSetConfig (93): electionTimeoutMillis (2000) must be greater than heartbeatIntervalMillis (2000) to allow at least one heartbeat before the election timeout expires
```

This makes every `antithesis_*` image build task fail its resmoke sanity check (fixture setup errors, resmoke exits with code 2).

This change explicitly sets `heartbeatIntervalMillis=1000` in the Antithesis block of `ReplicaSetFixture`, rather than raising `electionTimeoutMillis`, to preserve the intentionally aggressive 2s election timeout used to surface election bugs under Antithesis (SERVER-81475). A shorter heartbeat interval also increases heartbeat traffic, consistent with that goal.