Remove slack data from conic model

[andrewrosemberg]

Summary
The current conic model representation still stores (or relies on) slack variable data that should be internal or eliminated. This creates confusion and potential inconsistency when copying / transforming models for DiffOpt. Cleaning this up will simplify the model data structures, reduce duplication, and lower maintenance overhead.

Proposed Approach

• Identify where slack-related data structures are stored in the conic model.
• Refactor to remove slack data storage.
• Update any code that constructs, copies, or differentiates the conic model to use the refactored representation.
• Adjust tests to reflect new structure.

TODO from: #281