Add DownloadableMixin to endpoint package __all__

The mixin was imported in __init__.py but omitted from __all__, making
the export implicit. Add it explicitly so the public surface matches the
import statement.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jacalata

tableauserverclient/server/endpoint/__init__.py

@@ -40,6 +40,7 @@
     "DataAlerts",
     "Databases",
     "Datasources",
+    "DownloadableMixin",
     "QuerysetEndpoint",
     "MissingRequiredFieldError",
     "Endpoint",