Proper exception handling for RegisterPrivateAccessor()

- Add proper handling when a property’s backing field cannot be inferred or the unsafe accessor cannot be generated.
- Add DesignStrings methods and entries to .resx

Fixes #35219

Author: kawasaniac

src/EFCore.Design/Properties/DesignStrings.Designer.cs

@@ -156,6 +156,9 @@
   <data name="CompilationMustBeLoaded" xml:space="preserve">
     <value>A compilation must be loaded.</value>
   </data>
+  <data name="CompiledModelBackingFieldNotFound" xml:space="preserve">
+    <value>Backing field for property '{entityType}.{propertyName}' could not be inferred. Use a matching field name or configure with HasField().</value>
+  </data>
   <data name="CompiledModelConstructorBinding" xml:space="preserve">
     <value>The entity type '{entityType}' has a custom constructor binding. Compiled model can't be generated, because custom constructor bindings are not supported. Configure the custom constructor binding in '{customize}' in a partial '{className}' class instead.</value>
   </data>
@@ -168,6 +171,9 @@
   <data name="CompiledModelQueryFilter" xml:space="preserve">
     <value>The entity type '{entityType}' has a query filter configured. Compiled model can't be generated, because query filters are not supported.</value>
   </data>
+  <data name="CompiledModelUnsafeAccessorNull" xml:space="preserve">
+    <value>Unsafe accessor for property '{entityType}.{propertyName}' could not be generated. Ensure the property can be accessed or has a matching backing field.</value>
+  </data>
   <data name="CompiledModelValueGenerator" xml:space="preserve">
     <value>The property '{entityType}.{property}' has a value generator configured. Use '{method}' to configure the value generator factory type.</value>
   </data>

src/EFCore.Design/Properties/DesignStrings.resx

@@ -1769,7 +1769,9 @@ private void RegisterPrivateAccessors(
         Dictionary<Type, HashSet<MemberInfo>> unsafeAccessorTypes,
         ref Dictionary<MemberInfo, QualifiedName>? memberAccessReplacements)
     {
-        var member = property.GetMemberInfo(forMaterialization, forSet);
+        var member = property.GetMemberInfo(forMaterialization, forSet) ?? throw new InvalidOperationException(
+                DesignStrings.CompiledModelBackingFieldNotFound(property.DeclaringType.ShortName(), property.Name));
+
         switch (member)
         {
             case FieldInfo field:
@@ -1799,7 +1801,9 @@ private void RegisterPrivateAccessors(
         }
 
         memberAccessReplacements ??= [];
-        var methodName = LinqToCSharpSyntaxTranslator.GetUnsafeAccessorName(member);
+
+        var methodName = LinqToCSharpSyntaxTranslator.GetUnsafeAccessorName(member) ?? throw new InvalidOperationException(
+                DesignStrings.CompiledModelUnsafeAccessorNull(property.DeclaringType.ShortName(), property.Name));
 
         var declaringType = member.DeclaringType!;
         if (declaringType.IsGenericType