[python] Add payload/xml mock API coverage for PR #10558 union formatting scenario

[Copilot]

PR #10558 changed payload/xml scenario shape formatting (XmlErrorValue.get() union layout). This updates python mock API coverage to explicitly validate behavior for that scenario path in both sync and async clients.

• Spec alignment

  • Bump @typespec/http-specs to a version containing the updated payload/xml Spector case (0.1.0-alpha.37-dev.6).

• Mock API test coverage (shared sync + async)

  • Extend existing shared tests:
    • tests/mock_api/shared/test_payload_xml.py
    • tests/mock_api/shared/asynctests/test_payload_xml_async.py
  • Add focused assertions for xml_error_value.get() error response content type to ensure XML response handling remains stable with the updated union scenario shape.

• Changelog

  • Add .chronus/changes/* entry for @typespec/http-client-python (changeKind: internal).

with pytest.raises(core_library.exceptions.HttpResponseError) as ex:
    client.xml_error_value.get()

assert ex.value.response.headers["content-type"].startswith("application/xml")