[ishaaaan17]

summary of changes

fixes #21274.

when an RNTuple collection proxy field (for example std::set or std::map) was created for a type that has no dictionary, ROOT hit an assertion failure in TGenCollectionProxy.cxx:401 (R__ASSERT(e->IsValid())), or failed while inspecting the collection proxy.

this change:

1. checks classp->HasDictionary() in the constructor of RProxiedCollectionField, before the collection proxy is generated, and throws a ROOT::RException with the name of the missing type ("<type> has no dictionary entry available").
2. adds a unit test in tree/ntuple/test/ntuple_types.cxx under TEST(RNTuple, TVirtualCollectionProxy). it checks that a collection type without a loaded dictionary (std::map<double, double>) throws a ROOT::RException with this error message.

checklist

• code style checked with .clang-format
• unit test added
• follows the minimal diff guidelines (12 lines added, 0 deleted)

ai disclosure

Assisted-by: antigravity:gemini. i used an ai assistant to follow the collection proxy generation call stack in RProxiedCollectionField and TGenCollectionProxy. i checked the dictionary lookup against TClass::HasDictionary(), wrote the unit test and the exception check, and checked the formatting with .clang-format.