[python] add imports for readonly and constant props during msrest model generation

.chronus/changes/python-msrestDatetimeImportReadonly-2025-7-28-16-44-51.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-client-python"
+---
+
+Add imports for readonly and constant props for msrest model generation

packages/http-client-python/generator/pygen/codegen/serializers/model_serializer.py

@@ -158,6 +158,16 @@ def imports(self) -> FileImport:
                         called_by_property=True,
                     )
                 )
+            for prop in model.properties:
+                if prop.readonly:
+                    # it will be defined in the __init__ so we need to import it
+                    file_import.merge(
+                        prop.imports(
+                            serialize_namespace=self.serialize_namespace,
+                            serialize_namespace_type=NamespaceType.MODEL,
+                            called_by_property=True,
+                        )
+                    )
 
         return file_import