[plantuml parser] cover template special case

[hoe-jo]

No description provided.

[melodyoncode]

Looks good to me.

Nit: Could we add an integration test for this?

The new unit tests cover ResolvedType::Dependent, but the key behavior depends on how libclang exposes real C++ types. It would be useful to cover both sides of decltype(...):

• template-dependent decltype, e.g. a base class using decltype(is_x_impl(std::declval<T>())), which cannot be resolved without template instantiation and should not panic;
• non-template/concrete decltype, e.g. following a concrete API return type, which should still resolve to the actual return type when libclang provides canonical type information.

That would make the intended scope of the change much clearer and guard against accidentally treating all decltype usages as dependent.