fix(llm): key migration cost overrides by targetModelSlug

The override dict should be keyed by the TARGET model slug (where nodes
migrated TO), not the source model slug. This ensures the custom cost
is applied when building costs for the model that nodes are actually using.

autogpt_platform/backend/backend/data/block_cost_config.py

@@ -94,8 +94,10 @@ async def _build_llm_costs_from_registry() -> list[BlockCost]:
                 "customCreditCost": {"not": None},
             }
         )
+        # Key by targetModelSlug since that's the model nodes are now using
+        # after migration. The custom cost applies to the target model.
         migration_overrides = {
-            migration.sourceModelSlug: migration.customCreditCost
+            migration.targetModelSlug: migration.customCreditCost
             for migration in active_migrations
             if migration.customCreditCost is not None
         }