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

[Copilot]

Adds Python test coverage for the XML model with enum property introduced in #9660.

Changes

• Sync test (test_payload_xml.py): Tests GET/PUT operations for ModelWithEnum with Status enum
• Async test (test_payload_xml_async.py): Async variant of the enum test
• Changelog: Added internal change entry

Test Example

def test_model_with_enum(client: XmlClient):
    model = ModelWithEnum(status="success")
    assert client.model_with_enum_value.get() == model
    client.model_with_enum_value.put(model)

The tests validate serialization/deserialization of XML payloads containing extensible enum values ("pending", "success", "error").

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[http-client-python] Add test case for XML model with enum</issue_title>
<issue_description>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 #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</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [http-client-python] Add test case for XML model with enum #9662

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[annatisch]

Let's use the generated enum rather than the raw string, e.g. status=Status.SUCCESS

[annatisch]

Let's use the generated enum rather than the raw string, e.g. status=Status.SUCCESS