fix(reduce_sum_static): wrap Args with ref_type_t to materialize Eigen expressions

reduce_sum has been wrapping shared Args with ref_type_t<Args&&>... since
v4.8+ so that Eigen expression-template temporaries (e.g. M.row(0),
matrix.col(i)) materialize before being handed off to TBB's parallel
reducer. reduce_sum_static's STAN_THREADS branch was not updated in the
same pass and still forwarded plain Args..., so callers passing a
temporary Eigen expression as a shared argument could get a dangling
reference once the temporary went out of scope.

Mirror the reduce_sum wrapping exactly:

    internal::reduce_sum_impl<..., ref_type_t<Args&&>...>()(...)

No other changes. ref_type_t is already in scope via
stan/math/prim/meta.hpp (already included).

Closes #3304

Author: tmchow

stan/math/prim/functor/reduce_sum_static.hpp

@@ -50,9 +50,9 @@ inline auto reduce_sum_static(Vec&& vmapped, int grainsize, std::ostream* msgs,
 
 #ifdef STAN_THREADS
   return internal::reduce_sum_impl<ReduceFunction, void, return_type, Vec,
-                                   Args...>()(std::forward<Vec>(vmapped), false,
-                                              grainsize, msgs,
-                                              std::forward<Args>(args)...);
+                                   ref_type_t<Args&&>...>()(
+      std::forward<Vec>(vmapped), false, grainsize, msgs,
+      std::forward<Args>(args)...);
 #else
   if (vmapped.empty()) {
     return return_type(0);