[CORE] Fixes composed schema discriminator map (#4906)

* Adds addComposedMappedModels and testComposedSchemaOneOfDiscriminatorMap

* Requires that discriminators be required properties

* Strengthens discriminaotr validation, adds better error messages, adds schema oneof samples

* Adds oneOf and anyOf invalidDiscriminator tests

* Updates incorrect addOneOfInterfaceModel invocation

* Runs ensure-up-to-date

* Adds updates from Sebastien Rosset

* Removes newlines

* Uses df.isString

* Fixes tests be correctly setting df.isString

* Updates discriminatorExplicitMappingVerbose description per PR feedback

* Adds description of how mappedModels is populated

* Adds the suggestion exception raising when a MappedModel mappingName is null

* Actually resolves merge conflicts

* Switches two methods to package private because they are needed for testing

* Allow nulls in MappedModel.getMappingName

* Updates CLI flag name to legacyDiscriminatorBehavior, default=true

Co-authored-by: William Cheng <wing328hk@gmail.com>

samples/client/petstore/python-asyncio/petstore_api/api_client.py

@@ -625,9 +625,12 @@ class ApiClient(object):
         :param klass: class literal.
         :return: model object.
         """
+        has_discriminator = False
+        if (hasattr(klass, 'get_real_child_model')
+                and klass.discriminator_value_class_map):
+            has_discriminator = True
 
-        if not klass.openapi_types and not hasattr(klass,
-                                                   'get_real_child_model'):
+        if not klass.openapi_types and has_discriminator is False:
             return data
 
         kwargs = {}
@@ -641,7 +644,7 @@ class ApiClient(object):
 
         instance = klass(**kwargs)
 
-        if hasattr(instance, 'get_real_child_model'):
+        if has_discriminator:
             klass_name = instance.get_real_child_model(data)
             if klass_name:
                 instance = self.__deserialize(data, klass_name)