Fix well model slave group data exchange for history mode reservoir coupling

In history mode reservoir coupling, no GRUPSLAV keyword is defined because
rate allocation is not needed (wells use WCONHIST with imposed rates).
This caused assertion failures when the well model attempted to exchange
slave group data with numSlaveGroups == 0.

Changes:
- Skip receiving slave group production/injection data when numSlaveGroups == 0
- Skip sending slave group data when numSlaveGroups == 0

opm/simulators/flow/rescoup/ReservoirCouplingMasterReportStep.cpp

@@ -120,7 +120,14 @@ receiveInjectionDataFromSlaves()
     this->logger().info("Receiving injection data from slave processes");
     for (unsigned int i = 0; i < num_slaves; i++) {
         auto num_slave_groups = this->numSlaveGroups(i);
-        assert( num_slave_groups > 0 );
+        if (num_slave_groups == 0) {
+            // History mode: no slave groups defined, skip data exchange
+            this->logger().info(fmt::format(
+                "Slave {} has no slave groups (history mode), skipping injection data exchange",
+                this->slaveName(i)
+            ));
+            continue;
+        }
         std::vector<SlaveGroupInjectionData> injection_data(num_slave_groups);
         if (this->comm().rank() == 0) {
             if (this->slaveIsActivated(i)) {
@@ -169,7 +176,14 @@ receiveProductionDataFromSlaves()
     this->logger().info("Receiving production data from slave processes");
     for (unsigned int i = 0; i < num_slaves; i++) {
         auto num_slave_groups = this->numSlaveGroups(i);
-        assert( num_slave_groups > 0 );
+        if (num_slave_groups == 0) {
+            // History mode: no slave groups defined, skip data exchange
+            this->logger().info(fmt::format(
+                "Slave {} has no slave groups (history mode), skipping production data exchange",
+                this->slaveName(i)
+            ));
+            continue;
+        }
         std::vector<SlaveGroupProductionData> production_data(num_slave_groups);
         if (this->comm().rank() == 0) {
             if (this->slaveIsActivated(i)) {

opm/simulators/wells/rescoup/RescoupSendSlaveGroupData.cpp

@@ -299,6 +299,10 @@ sendSlaveGroupProductionDataToMaster_() const
 {
     auto& rescoup_slave = this->reservoir_coupling_slave_;
     auto num_slave_groups = rescoup_slave.numSlaveGroups();
+    if (num_slave_groups == 0) {
+        // History mode: no slave groups defined, nothing to send
+        return;
+    }
     std::vector<SlaveGroupProductionData> production_data;
     for (std::size_t group_idx = 0; group_idx < num_slave_groups; ++group_idx) {
         // NOTE: We have to send production data even if the slave group is not a production group,
@@ -315,6 +319,10 @@ sendSlaveGroupInjectionDataToMaster_() const
 {
     auto& rescoup_slave = this->reservoir_coupling_slave_;
     auto num_slave_groups = rescoup_slave.numSlaveGroups();
+    if (num_slave_groups == 0) {
+        // History mode: no slave groups defined, nothing to send
+        return;
+    }
     std::vector<SlaveGroupInjectionData> injection_data;
     for (std::size_t group_idx = 0; group_idx < num_slave_groups; ++group_idx) {
         // NOTE: We would like to only send injection data only if the master group is an injector,