Move Enzyme tests to NoPre test group

This change addresses CI failures on Julia prerelease versions by:
- Creating a new 'nopre' test group for tests that use Enzyme
- Adding the nopre group to CI configuration for NonlinearSolve and SimpleNonlinearSolve
- Excluding nopre tests from running on Julia 'pre' version in CI
- Creating enzyme_tests.jl with example Enzyme tests that skip on prerelease

All existing Enzyme tests in subpackages already have VERSION.prerelease
checks, but this change provides an additional CI-level control to prevent
these tests from running on prerelease Julia versions.

Fixes test failures seen in https://github.com/SciML/NonlinearSolve.jl/actions/runs/16851672503/job/47738829623

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Author: ChrisRackauckas

.github/workflows/CI_NonlinearSolve.yml

@@ -36,13 +36,18 @@ jobs:
           - downstream
           - wrappers
           - misc
+          - nopre
         version:
           - "1"
           - "lts"
           - "pre"
         os:
           - ubuntu-latest
           - macos-latest
+        exclude:
+          # Don't run nopre tests on prerelease Julia
+          - group: nopre
+            version: "pre"
     steps:
       - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v2

.github/workflows/CI_SimpleNonlinearSolve.yml

@@ -34,6 +34,7 @@ jobs:
           - core
           - adjoint
           - alloc_check
+          - nopre
     steps:
       - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v2

test/enzyme_tests.jl

@@ -0,0 +1,54 @@
+@testitem "PolyAlgorithms with Enzyme" tags=[:nopre] skip=!isempty(VERSION.prerelease) begin
+    using ADTypes
+
+    # Only run these tests on non-prerelease Julia versions
+    @info "Running Enzyme tests (Julia $(VERSION))"
+    
+    cache = zeros(2)
+    function f(du, u, p)
+        cache .= u .* u
+        du .= cache .- 2
+    end
+    u0 = [1.0, 1.0]
+    probN = NonlinearProblem{true}(f, u0)
+
+    # Test with AutoEnzyme for autodiff
+    if isempty(VERSION.prerelease)
+        using Enzyme
+        sol = solve(probN, RobustMultiNewton(; autodiff = AutoEnzyme()))
+        @test SciMLBase.successful_retcode(sol)
+        
+        sol = solve(
+            probN, FastShortcutNonlinearPolyalg(; autodiff = AutoEnzyme()); abstol = 1e-9
+        )
+        @test SciMLBase.successful_retcode(sol)
+    end
+end
+
+@testitem "ForwardDiff with Enzyme backend" tags=[:nopre] skip=!isempty(VERSION.prerelease) begin
+    using ForwardDiff, ADTypes
+    
+    # Only run these tests on non-prerelease Julia versions
+    @info "Running ForwardDiff-Enzyme integration tests (Julia $(VERSION))"
+    
+    test_f!(du, u, p) = (@. du = u^2 - p)
+    test_f(u, p) = (@. u^2 - p)
+    
+    function solve_oop(p)
+        solve(NonlinearProblem(test_f, 2.0, p), NewtonRaphson(; autodiff = AutoEnzyme())).u
+    end
+    
+    if isempty(VERSION.prerelease)
+        using Enzyme
+        
+        # Test scalar AD with Enzyme backend
+        for p in 1.0:0.1:10.0
+            sol = solve(NonlinearProblem(test_f, 2.0, p), NewtonRaphson(; autodiff = AutoEnzyme()))
+            if SciMLBase.successful_retcode(sol)
+                gs = abs.(ForwardDiff.derivative(solve_oop, p))
+                gs_true = abs.(1 / (2 * √p))
+                @test abs.(gs) ≈ abs.(gs_true) atol=1e-5
+            end
+        end
+    end
+end