fix: fail delete_collection clearly in cloud mode — the API has no folder deletion

The server exposes only POST /folder and GET /folders (verified against the
full route table and its git history — a DELETE /folder route never
existed; the 0.2.x SDK accordingly shipped create_folder/list_folders but
no delete). The DELETE /folder/{id}/ call could only 404, surfacing as a
misleading 'Not Found' CloudAPIError while leaving the stale folder id in
the cache. Raise a clear not-supported error pointing at the dashboard;
keep the idempotent no-op for missing collections and folderless plans.

pageindex/backend/cloud.py

@@ -205,13 +205,14 @@ class CloudBackend:
         except CollectionNotFoundError:
             return  # already gone — delete is idempotent
         if folder_id:
-            self._request("DELETE", f"/folder/{self._enc(folder_id)}/")
-            # Drop the cached id so a later same-name op re-resolves instead of
-            # reusing the now-deleted folder_id. Only when it was a REAL id —
-            # if folder_id was falsy, the cache holds the "folders unavailable
-            # on this plan" None sentinel, which must survive so we don't
-            # re-issue a doomed GET /folders/ on the next call.
-            self._folder_id_cache.pop(name, None)
+            # The cloud API has no folder-deletion endpoint (only POST /folder
+            # and GET /folders exist) — fail clearly instead of issuing a
+            # request that can only 404.
+            raise PageIndexError(
+                f"Deleting a cloud collection is not supported by the PageIndex "
+                f"API — delete folder '{name}' in the dashboard "
+                "(https://dash.pageindex.ai) instead."
+            )
 
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