Improve parameter checks or allow mixing parameters and functions when setting sensitivities

[andrewrosemberg]

Summary
When setting sensitivities (sensibs) the current API inconsistently handles mixing parameters and functions. Need to either enforce clearer validation with better errors or support the mixed usage.

Decision Needed
Do we (A) strictly enforce separation with clear errors, or (B) support mixing? This issue covers implementing whichever path is chosen (may split later if needed).

TODO from: #281