Add support for modifying design responses.

### 📝 Description of the feature

Add support for modifying design responses locally via Design class.

### 💡 Steps for implementing the feature

_No response_

### 🔗 Useful links and references

_No response_

Start by reviewing the Design class and the linked pull request #516, which is already implementing this request. Determine how design responses are represented and modified locally there; done means the requested modification support is covered by the pull request and its validation.

Written by the indexing model from the issue text.