A forward-sensitivity run refused partway arrives as a plain SimulationError, told from a solver failure only by its text #1435

#1336 (#1313) gives two of bngsim's refusals by name types of their own. #1411 (#1279) adds a C++ bngsim::SensitivityUnsupported, which the pybind11 layer maps to SensitivityUnsupportedError, for the one refusal it adds.

The rest of the refusals a sensitivity run reaches partway are still untyped. These are runs that CVODE starts and then refuses, because something the forward-sensitivity path can't follow happens during the run, for example:

• a parameter's column moves an event's crossing;
• a column moves a counter species;
• the run restarts;
• a state-dependent rate-law switch crosses;
• two events fire on one instant.

src/cvode_simulator.cpp builds 45 such messages, each starting Forward sensitivity:. It throws all 45 as std::runtime_error: 43 where the message is built, and two after keeping it as a reason (L9892). Examples: L6355, L6825, L7207. The count is the same with #1411 merged. run() wraps each one as SimulationError("Simulation failed: …").

A caller that has to tell these refusals from a crash reads the text. The path-agreement harness's _declined counts a SimulationError as a refusal when its message contains "forward sensitivity" or "forward-sensitivity run" (#1229's class). There, a reworded message turns a refusal into a crash, as it did for the two refusals #1313 types.

Fix: throw #1411's SensitivityUnsupported for the 45 messages, once #1411 lands. _declined then needs only its isinstance.

One thing to decide for all 45 at once: SensitivityUnsupportedError subclasses BngsimError and ValueError, not SimulationError. So once these are typed, an except SimulationError around a sensitivity run stops catching them. #1411 made that choice for its one refusal. A subclass of both SimulationError and SensitivityUnsupportedError would keep both except clauses working.

Found in the review of #1336 (its low 5).