refactor: use discriminated and packed types from generation

[pyphilia]

Since the backend returns discriminated generated types Item and PackedItem, I've changed the codebase to use these types directly.

• To match with the backend, Item is the discriminated simple item. GenericItem is the base non-discriminated item.
• Because the factories, some functions, types come from sdk, I need to cast them.

[sonarqubecloud]

Quality Gate passed

Issues
20 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
38.8% Duplication on New Code

See analysis details on SonarQube Cloud