[ntuple] Add inner field type check for std::map fields

Inner fields *must* be of type `RPairField`. Ideally this should be done
at compile time through the constructor argument type, but this is still
causing issues and needs further investigation. Thus, this runtime check
is added as a temporary solution.

tree/ntuple/v7/src/RField.cxx

@@ -2682,6 +2682,9 @@ ROOT::Experimental::RMapField::RMapField(std::string_view fieldName, std::string
                                          std::unique_ptr<Detail::RFieldBase> itemField)
    : RProxiedCollectionField(fieldName, typeName, TClass::GetClass(std::string(typeName).c_str()))
 {
+   if (!dynamic_cast<RPairField *>(itemField.get()))
+      throw RException(R__FAIL("RMapField inner field type must be of RPairField"));
+
    fItemClass = fProxy->GetValueClass();
    fItemSize = fItemClass->GetClassSize();
 

tree/ntuple/v7/test/ntuple_types.cxx

@@ -455,6 +455,11 @@ TEST(RNTuple, StdMap)
    EXPECT_THROW(RFieldBase::Create("myInvalidMap", "std::map<char>").Unwrap(), RException);
    EXPECT_THROW(RFieldBase::Create("myInvalidMap", "std::map<char, std::string, int>").Unwrap(), RException);
 
+   auto invalidInnerField = RFieldBase::Create("someIntField", "int").Unwrap();
+   EXPECT_THROW(std::make_unique<ROOT::Experimental::RMapField>("myInvalidMap", "std::map<char, int>",
+                                                                std::move(invalidInnerField)),
+                RException);
+
    FileRaii fileGuard("test_ntuple_rfield_stdmap.root");
    {
       auto model = RNTupleModel::Create();