fix: add test coverage for optional permalink in EntityResponse schema

- Add comprehensive test for None permalink validation in EntityResponse
- Ensures schema properly handles markdown files without explicit permalinks
- Addresses GitHub issue #170 validation errors during edit operations
- Test validates that permalink=None doesn't cause ValidationError

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/schemas/response.py

@@ -131,7 +131,7 @@ class EntityResponse(SQLAlchemyModel):
     }
     """
 
-    permalink: Permalink
+    permalink: Optional[Permalink]
     title: str
     file_path: str
     entity_type: EntityType