Handle error when type parameter of mapped type uses private type

Fixes #30201

src/compiler/diagnosticMessages.json

@@ -2927,6 +2927,10 @@
         "category": "Error",
         "code": 4102
     },
+    "Type parameter '{0}' of exported mapped object type is using private name '{1}'.": {
+        "category": "Error",
+        "code": 4103
+    },
 
     "The current host does not support the '{0}' option.": {
         "category": "Error",

src/compiler/transformers/declarations/diagnostics.ts

@@ -391,6 +391,10 @@ namespace ts {
                     diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_interface_has_or_is_using_private_name_1;
                     break;
 
+                case SyntaxKind.MappedType:
+                    diagnosticMessage = Diagnostics.Type_parameter_0_of_exported_mapped_object_type_is_using_private_name_1;
+                    break;
+
                 case SyntaxKind.ConstructorType:
                 case SyntaxKind.ConstructSignature:
                     diagnosticMessage = Diagnostics.Type_parameter_0_of_constructor_signature_from_exported_interface_has_or_is_using_private_name_1;