[RF][HF] Check for consistent datasets across channels before combining

This makes problems like the one reported in #21066 easier to debug by
the user, because there will be a meaningful error message like this
instead of a crash:
```txt
[#2] FATAL:HistFactory -- ERROR: Inconsistent datasets across channel workspaces.
Workspace for channel "SR_3L_0RC_binary_binary_3L_0RC" does not match the datasets in channel "SR_2L_mva_2L_multiclass_ttll".
  Missing datasets:
    - obsData
All channel workspaces must contain exactly the same datasets.
terminate called after throwing an instance of 'RooStats::HistFactory::hf_exc'
  what():  HistFactory - Exception
```

(cherry picked from commit e37fefd)

Author: guitargeek

roofit/histfactory/src/HistoToWorkspaceFactoryFast.cxx

@@ -317,6 +317,80 @@ Arg_t &emplace(RooWorkspace &ws, std::string const &name, Args_t &&...args)
    return *dynamic_cast<Arg_t *>(ws.arg(name));
 }
 
+
+/// Check whether all channel workspaces contain consistent datasets.
+///
+/// This function compares the datasets stored in each channel workspace against
+/// those in the first workspace.
+///
+/// \param chs Vector of channel workspaces to compare (first is the reference).
+/// \param ch_names Names of the channels, used for error reporting.
+/// \param allowedInconsistent Dataset names that are allowed to differ between channels.
+///
+/// \return A pair consisting of:
+///   - bool: true if all channels are consistent (after ignoring allowed datasets),
+///           false otherwise.
+///   - std::string: empty if consistent; otherwise, a detailed error message
+///                  describing the inconsistencies.
+
+std::pair<bool, std::string> isChannelDataConsistent(std::vector<std::unique_ptr<RooWorkspace>> const &chs,
+                                                     std::vector<std::string> const &ch_names,
+                                                     std::set<std::string> const &allowedInconsistent)
+{
+   // Collect the reference list of dataset names from the first workspace
+   std::set<std::string> referenceDataNames;
+   for (RooAbsData *data : chs[0]->allData()) {
+      referenceDataNames.insert(data->GetName());
+   }
+
+   // Check that all other workspaces have the same datasets
+   for (std::size_t i = 1; i < chs.size(); ++i) {
+      std::set<std::string> thisDataNames;
+      for (RooAbsData *data : chs[i]->allData()) {
+         thisDataNames.insert(data->GetName());
+      }
+
+      // Find missing and extra datasets in this workspace
+      std::vector<std::string> missing;
+      std::vector<std::string> extra;
+      std::set_difference(referenceDataNames.begin(), referenceDataNames.end(), thisDataNames.begin(),
+                          thisDataNames.end(), std::back_inserter(missing));
+      std::set_difference(thisDataNames.begin(), thisDataNames.end(), referenceDataNames.begin(),
+                          referenceDataNames.end(), std::back_inserter(extra));
+
+      // Remove allowed inconsistencies
+      auto isAllowed = [&](std::string const &name) { return allowedInconsistent.count(name) != 0; };
+
+      missing.erase(std::remove_if(missing.begin(), missing.end(), isAllowed), missing.end());
+      extra.erase(std::remove_if(extra.begin(), extra.end(), isAllowed), extra.end());
+
+      if (!missing.empty() || !extra.empty()) {
+         std::stringstream errMsg;
+         errMsg << "ERROR: Inconsistent datasets across channel workspaces.\n"
+                << "Workspace for channel \"" << ch_names[i] << "\" does not match "
+                << "the datasets in channel \"" << ch_names[0] << "\".\n";
+
+         if (!missing.empty()) {
+            errMsg << "  Missing datasets:\n";
+            for (const auto &name : missing) {
+               errMsg << "    - " << name << "\n";
+            }
+         }
+
+         if (!extra.empty()) {
+            errMsg << "  Extra datasets:\n";
+            for (const auto &name : extra) {
+               errMsg << "    - " << name << "\n";
+            }
+         }
+
+         errMsg << "All channel workspaces must contain exactly the same datasets.\n";
+         return {false, errMsg.str()};
+      }
+   }
+   return {true, ""};
+}
+
 } // namespace
 
 /// Create observables of type RooRealVar. Creates 1 to 3 observables, depending on the type of the histogram.
@@ -1537,6 +1611,17 @@ RooArgList HistoToWorkspaceFactoryFast::createObservables(const TH1 *hist, RooWo
     combined->defineSet("observables",{obsList, channelCat}, /*importMissing=*/true);
     combined_config->SetObservables(*combined->set("observables"));
 
+    // Check if the channel datasets are consistent
+    {
+      bool isConsistent = false;
+      std::string errMsg;
+      std::set<std::string> allowedInconsistent{"asimovData"};
+      std::tie(isConsistent, errMsg) = isChannelDataConsistent(chs, ch_names, allowedInconsistent);
+      if (!isConsistent) {
+        cxcoutFHF << errMsg;
+        throw hf_exc();
+      }
+    }
 
     // Now merge the observable datasets across the channels
     for(RooAbsData * data : chs[0]->allData()) {