032: add aclose() to SupersetClient (was missing, used in dashboard detail routes)

SupersetClientBase was missing aclose() method. AsyncSupersetClient had it
via override, but code creating SupersetClient directly (e.g. dashboard tasks
history route) would fail with AttributeError on await client.aclose().

backend/src/core/superset_client/_base.py

@@ -272,5 +272,12 @@ class SupersetClientBase:
             )
             return data
     # #endregion SupersetClientGetAllResources
+
+    # #region aclose [TYPE Function]
+    # @PURPOSE Close async HTTP transport.
+    # @POST Underlying AsyncAPIClient session is closed.
+    async def aclose(self) -> None:
+        await self.client.aclose()
+    # #endregion aclose
 # #endregion SupersetClientBase
 # #endregion SupersetClientBase