Return a fresh dict from Server.namespace so callers cannot pollute shared state

Copilot flagged that Server.namespace returned the module-level
_NAMESPACE_MAP directly, so a caller mutating the returned dict would
have polluted every other Server instance in the process. Old namespace-
handling code returned a per-instance dict, so no cross-instance
pollution was possible. Wrap in dict(...) to restore that behavior.

Author: jacalata

tableauserverclient/server/server.py

@@ -293,7 +293,7 @@ def baseurl(self):
 
     @property
     def namespace(self):
-        return _NAMESPACE_MAP
+        return dict(_NAMESPACE_MAP)
 
     @property
     def auth_token(self):