fix: data persistence bug introduced in #2433 (#2460)

The Type Checking fix from #2433 introduced a new issue that causes an
error while persisting to the database.
This PR should fix that while passing mypy checks. 

    if not isinstance(self.storage_client, GCSStorageClient)
                                           ^^^^^^^^^^^^^^^^
NameError: name 'GCSStorageClient' is not defined. Did you mean:
'BaseStorageClient'?

---------

Co-authored-by: Josh Hayes <[email protected]>

Author: mihidumh

backend/chainlit/data/chainlit_data_layer.py

@@ -26,6 +26,12 @@
 )
 from chainlit.user import PersistedUser, User
 
+# Import for runtime usage (isinstance checks)
+try:
+    from chainlit.data.storage_clients.gcs import GCSStorageClient
+except ImportError:
+    GCSStorageClient = None  # type: ignore[assignment,misc]
+
 if TYPE_CHECKING:
     from chainlit.data.storage_clients.gcs import GCSStorageClient
     from chainlit.element import Element, ElementDict
@@ -202,7 +208,10 @@ async def create_element(self, element: "Element"):
         if content is not None:
             content_disposition = (
                 f'attachment; filename="{element.name}"'
-                if not isinstance(self.storage_client, GCSStorageClient)
+                if not (
+                    GCSStorageClient is not None
+                    and isinstance(self.storage_client, GCSStorageClient)
+                )
                 else None
             )
             await self.storage_client.upload_file(