memory (sqla): Improve handling of RSM in get_items:

- update queries to handle correctly the `before=""` case and RSM order (older to newer)
  while having correct order when `max_items` without RSM is used (then order it newer to
  older).
- put RSM code path in a separated method for better readability.
- check values of `max_items` and `from_index` and raise exception is they are not
  positive
- when using `before` or `after`, check that the item ID used as reference exists, and
  raise an exception otherwise.
- updated docstring to reflect the new behaviour.
- update pubsub cache use of `get_items` with RSM, to adapt to new behaviour.

rel 476

libervia/backend/plugins/plugin_pubsub_cache.py

@@ -505,20 +505,15 @@ class PubsubCache:
                 node, max_items=max_items, order_by=extra.get(C.KEY_ORDER_BY)
             )
         else:
-            desc = False
-            if rsm_request.before == "":
-                before = None
-                desc = True
-            else:
-                before = rsm_request.before
+            # RSM request
             pubsub_items, metadata = await self.host.memory.storage.get_items(
                 node,
                 max_items=rsm_request.max,
-                before=before,
+                before=rsm_request.before,
                 after=rsm_request.after,
                 from_index=rsm_request.index,
                 order_by=extra.get(C.KEY_ORDER_BY),
-                desc=desc,
+                desc=False,
                 force_rsm=True,
             )