fix(core): correct cursor direction for descending pagination in list_agent_blocks_async (#9122)

The cursor-based pagination was not accounting for sort order. When using
descending order (the default), "after cursor X" should return items with
id < X (items that come after X in the descending result set), but the code
was using id > X which caused infinite loops in clients iterating through pages.

This fix adjusts the cursor comparison based on the sort order:
- ascending: after=id > X, before=id < X
- descending: after=id < X, before=id > X

Note: Other pagination methods (list_agent_sources_async, list_agent_tools_async,
list_agent_groups_async) may have the same issue and should be audited.

🐾 Generated with [Letta Code](https://letta.com)

Co-authored-by: Letta <noreply@letta.com>

letta/services/agent_manager.py

@@ -3046,10 +3046,19 @@ class AgentManager:
             )
 
             # Apply cursor-based pagination
-            if before:
-                query = query.where(BlockModel.id < before)
-            if after:
-                query = query.where(BlockModel.id > after)
+            # Note: cursor direction must account for sort order
+            # - ascending order: "after X" means id > X, "before X" means id < X
+            # - descending order: "after X" means id < X, "before X" means id > X
+            if ascending:
+                if before:
+                    query = query.where(BlockModel.id < before)
+                if after:
+                    query = query.where(BlockModel.id > after)
+            else:
+                if before:
+                    query = query.where(BlockModel.id > before)
+                if after:
+                    query = query.where(BlockModel.id < after)
 
             # Apply sorting - use id instead of created_at for core memory blocks
             if ascending: