[RF][HF] Persist ShapeFactor const-ness to XML

Sample::PrintXML now writes the "Const" attribute for ShapeFactors, so a
ShapeFactor that was marked constant via SetConstant() keeps that state
when a Measurement is written to XML and read back.

The read side and schema were already in place:
**ConfigParser::MakeShapeFactor** parses the "Const" attribute, and the
ShapeFactor element declares it in HistFactorySchema.dtd. Only the
writer was missing it, so the const-ness was silently lost on a PrintXML
round trip. The attribute is always emitted as "True"/"False", matching
how other booleans (NormalizeByTheory, StatError's Activate) are
written.

Unlike NormFactor, whose const-ness is handled at the Measurement level
via `<ParamSetting Const="True">`, ShapeFactor carries its own per-bin
constant flag (wired into the gamma parameters in
HistoToWorkspaceFactoryFast), which is why it needs to be persisted on
the element itself.

🤖 Done by AI.

(cherry picked from commit 3c8740f)

Author: guitargeek

roofit/histfactory/src/Measurement.cxx

@@ -1769,7 +1769,8 @@ void ShapeFactor::PrintXML(std::ostream &xml) const
    }
    xml << " Val=\"" << GetVal() << "\" "
        << " High=\"" << GetHigh() << "\" "
-       << " Low=\"" << GetLow() << "\" ";
+       << " Low=\"" << GetLow() << "\" "
+       << " Const=\"" << (IsConstant() ? std::string("True") : std::string("False")) << "\" ";
    xml << "  /> " << std::endl;
 }