Return tunable vector for Enzyme tangent instead of NamedTuple (#1358)

For EnzymeOriginator with SciMLStructure parameters, return the tunable
gradient vector directly from steadystatebackpass instead of the
Zygote-repacked NamedTuple. The NonlinearSolveBaseEnzymeExt reverse
rule uses SciMLStructures.replace! to accumulate it into the parameter
shadow, going through the proper SciMLStructures interface.

This avoids the NamedTuple broadcasting error and ensures all tangent
accumulation uses SciMLStructures.canonicalize/replace! rather than
making assumptions about the NamedTuple field structure.

Companion PR: NonlinearSolve.jl#879

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ChrisRackauckas

src/concrete_solve.jl

@@ -2418,16 +2418,26 @@ function SciMLBase._concrete_solve_adjoint(
                 Δtunables
         )
 
+        # For Enzyme with SciMLStructure parameters, return the tunable gradient
+        # vector directly instead of the Zygote-repacked NamedTuple. The Enzyme
+        # reverse rule in NonlinearSolveBaseEnzymeExt uses
+        # SciMLStructures.replace! to accumulate it into the parameter shadow.
+        dp_tangent = if originator isa SciMLBase.EnzymeOriginator && isscimlstructure(p)
+            dp
+        else
+            repack_adjoint(dp)[1]
+        end
+
         return if originator isa SciMLBase.TrackerOriginator ||
                 originator isa SciMLBase.ReverseDiffOriginator
             (
-                NoTangent(), NoTangent(), NoTangent(), repack_adjoint(dp)[1], NoTangent(),
+                NoTangent(), NoTangent(), NoTangent(), dp_tangent, NoTangent(),
                 ntuple(_ -> NoTangent(), length(args))...,
             )
         else
             (
                 NoTangent(), NoTangent(), NoTangent(),
-                NoTangent(), repack_adjoint(dp)[1], NoTangent(),
+                NoTangent(), dp_tangent, NoTangent(),
                 ntuple(_ -> NoTangent(), length(args))...,
             )
         end