Set ifrSenderVersion on outgoing aggregate commands dispatched to shards in addIfrFlagsToRequest, with the correct semantics:* A router (pure mongos) should stamp its own FCV string so downstream shards know the router's version.

• A shard that received ifrFlags from a router should forward the ifrSenderVersion it received rather than substituting its own, to avoid confusing the downstream shard's resolution logic.

The field is already declared in generic_argument.idl with forward_to_shards: true and ifrSenderVersion is already preserved through serializeForPassthrough in cluster_aggregate.cpp. The remaining work is to call request.setIfrSenderVersion(...) inside addIfrFlagsToRequest (src/mongo/db/pipeline/aggregation_request_helper.cpp) conditioned on whether the local IFRContext was wire-installed.