[http-client-python] Add test case for XML model with enum

Add a test case to https://github.com/microsoft/typespec/blob/main/packages/http-client-python/generator/test/generic_mock_api_tests/test_payload_xml.py for https://github.com/microsoft/typespec/pull/9660 which added a new test API for an XML payload with an enum.

Also add the equivalent async test to https://github.com/microsoft/typespec/blob/main/packages/http-client-python/generator/test/generic_mock_api_tests/asynctests/test_payload_xml_async.py

NOTE:

- DO add changelog
- DO format your changed files before commit