nanobind 2.13.0 changed failing `nb::cast<T>()` to raise `cast_error`
(an alias for `std::bad_cast`) for all failures, including `None`,
instead of the previous distinct exception types. The bindings relied on
catching `std::runtime_error` separately from `cast_error` to emit a
helpful "(None?)" hint, so that branch became dead code and the `None`
diagnostics regressed (e.g. "(std::bad_cast)" instead of "(None?)"),
breaking check-mlir Python tests.
Fix: Decide the `None` hint from the value via `is_none()` rather than
the exception type. This is robust across nanobind versions (2.10-2.13).
Fixes#205329
Assisted by: Claude
Maksim Levental
6a12b7c74b