Propagate initial values to reduced variables in CvxAttr2Constr

When CvxAttr2Constr creates reduced variables for dim-reducing attributes
(e.g., diag), the original variable's initial value was not being lowered
and assigned to the reduced variable. This caused NLP solvers to fail with
"Variable has no value" during initial point construction.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Transurgeon

cvxpy/reductions/cvx_attr2constr.py

@@ -217,6 +217,8 @@ def apply(self, problem):
 
                     reduced_var = Variable(n, var_id=var.id, **new_attr)
                     reduced_var.set_leaf_of_provenance(var)
+                    if var.value is not None:
+                        reduced_var.value = lower_value(var)
                     id2new_var[var.id] = reduced_var
                     obj = build_dim_reduced_expression(var, reduced_var)
                 elif new_var: