Add a new DAG status to differentciate between the data handling, mdoelo building and actually solving the model

This issue has no description.

The issue proposes new DAG statuses to distinguish data handling, model building, and solving, but provides no files, tests, entry points, or acceptance criteria. First map the existing DAG status flow and clarify the intended states and transitions with the maintainers. Done should include agreed status behavior and coverage for the affected workflow.

Written by the indexing model from the issue text.