[python] Add mock_api coverage for namespace-scoped @usage scenario

[Copilot]

This PR adds Python SDK mock API coverage for the new Azure Spector scenario introduced in Azure/typespec-azure PR #4493, where @usage is applied at a namespace level and propagates to nested types.
It ensures the generated usage client surface is exercised for the new namespaceModelSerializable operation in both sync and async test suites.

• Scenario alignment (Azure Spector update)

  • Updated @azure-tools/azure-http-specs in packages/http-client-python to a version containing Azure_ClientGenerator_Core_Usage_NamespaceUsage.

• Azure mock_api test coverage (sync + async)

  • Extended existing usage tests:
    • tests/mock_api/azure/test_azure_client_generator_core_usage.py
    • tests/mock_api/azure/asynctests/test_azure_client_generator_core_usage_async.py
  • Added test_namespace_model_serializable in both files to validate SDK invocation of the new namespace-usage route.

• Change log

  • Added Chronus entry for @typespec/http-client-python documenting the test coverage addition.

def test_namespace_model_serializable(client: UsageClient):
    namespace_model = models.NamespaceModel(name="test")
    client.namespace_usage.namespace_model_serializable(body=namespace_model)