[RF] Correctly normalize max possible val in generation with proto data

The code branch without a proto data in `RooGenContext` already did
this, and now it's also done when using proto dataset generation.

Closes #12286.

roofit/roofit/test/testRooKeysPdf.cxx

 // Tests for the RooKeysPdf and friends
 // Authors: Jonas Rembser, CERN  07/2022
 
-#include <RooRealVar.h>
+#include <RooGenericPdf.h>
 #include <RooKeysPdf.h>
 #include <RooNDKeysPdf.h>
+#include <RooPlot.h>
+#include <RooRealVar.h>
 
 #include "gtest/gtest.h"
 
@@ -54,3 +56,29 @@ TEST(RooKeysPdf, WeightedDataset)
       xVal += 0.1;
    }
 }
+
+// Test generation with proto data, covering GitHub issue #12286.
+TEST(RooKeysPdf, GenerationWithProtoData)
+{
+   using namespace RooFit;
+
+   RooRealVar x{"x", "", 0, 1};
+   RooGenericPdf pdfX{"pdf_x", "x", {x}};
+
+   std::unique_ptr<RooDataSet> dtBase{pdfX.generate(x, 10000)};
+
+   RooKeysPdf pdfKeys{"pdf_keys", "", x, *dtBase, RooKeysPdf::MirrorBoth};
+
+   RooRealVar y{"y", "", 0, 1};
+   RooDataSet proto{"proto_y", "", y};
+   proto.add(y);
+
+   std::unique_ptr<RooDataSet> dtKeysWithProto{pdfKeys.generate(x, NumEvents(10000), ProtoData(proto))};
+
+   std::unique_ptr<RooPlot> frame{x.frame()};
+   dtKeysWithProto->plotOn(frame.get());
+   pdfKeys.plotOn(frame.get());
+
+   // If the dataset generation worked, the chi-square is not too terrible
+   EXPECT_LE(frame->chiSquare(), 2.0);
+}

roofit/roofitcore/src/RooGenContext.cxx

@@ -240,7 +240,8 @@ RooGenContext::RooGenContext(const RooAbsPdf &model, const RooArgSet &vars,
       coutI(Generation) << "RooGenContext::ctor() prototype data provided, and "
                         << "model supports analytical maximum finding in the full phase space: "
                         << "can provide analytical pdf maximum to numeric generator" << endl ;
-      _maxVar = new RooRealVar("funcMax","function maximum",_pdfClone->maxVal(maxFindCode)) ;
+       double maxVal = _pdfClone->maxVal(maxFindCode) / _pdfClone->getNorm(&allVars);
+      _maxVar = new RooRealVar("funcMax", "function maximum", maxVal);
     } else {
       maxFindCode = _pdfClone->getMaxVal(_otherVars) ;
       if (maxFindCode != 0) {