Disaggregated config-server transitions restart nodes with --configsvr. The ContinuousTransition hook subsequently adds or removes configsvr: true in the persisted replica-set config when reconfiguring as CSRS or as a RS.  Then, the hook submits replSetReconfig.

DSC parses that request through DisaggReplSetConfig. Its schema does not support the legacy configsvr field, causing InvalidReplicaSetConfig.

As part of this ticket, we have to add a fixture capability so standard replica sets retain legacy config reconfiguration, while DisaggReplicaSetFixture skips only the persisted-config mutation and continues the existing rolling --configsvr restarts.