[Bug]: Guard unsafe casts on `GetEObject(...)` results

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: Several SetProperties implementations cast the result of Resource.GetEObject(...) directly to a concrete type. GetEObject can fail to resolve (e.g. incomplete/cross-resource models), producing a NullReferenceException or InvalidCastException with no actionable message.
• Affected files: ECoreNetto/ModelElement/NamedElement/Classifier/EClass.cs:186, ECoreNetto/ModelElement/NamedElement/ETypedElement.cs:139, ECoreNetto/ModelElement/NamedElement/TypedElement/StructuralFeature/EReference.cs:122
• Proposed solution: Resolve into the base type, null-check, and either record a diagnostic (see B3) or throw a descriptive exception naming the unresolved fragment. Consider a small helper on Resource/EObject that resolves-and-validates a typed reference.
• Acceptance criteria:
  • Unresolved references produce a clear, message-bearing error (or recorded diagnostic) rather than NullReference/InvalidCast.
  • Tests cover the unresolved and wrong-type cases for each site.