Fix v7 CI: iterate eachindex(sol.u) in dense_tests, source DiffEqDevTools in DelayDiffEq (#3446)

- test/regression/ode_dense_tests.jl

  Under RecursiveArrayTools v4 `eachindex(sol2)` returns the
  CartesianIndices over the full solution tensor (u_dims..., nsteps),
  not the 1:nsteps range it returned under v3. `sol2.u[i]` /
  `interpolation_results_2d[i]` then tried a 3-index lookup into
  `Vector{Matrix{Float64}}` and threw `BoundsError [i, j, k]`. Iterate
  `eachindex(sol2.u)` instead to drive the per-timestep comparison the
  test actually means. Fixes the Dense Tests errors in Regression_I on
  1/1.11/pre.

- lib/DelayDiffEq/Project.toml

  Add `DiffEqDevTools = {path = "../DiffEqDevTools"}` to `[sources]`.
  DelayDiffEq's test target pulls in path-sourced StochasticDiffEqLowOrder
  which now pins RecursiveArrayTools v4 (via the v7 SciMLBase/RAT bump).
  All currently-registered DiffEqDevTools versions restrict RAT to < 4, so
  the env fails to resolve with ``Unsatisfiable requirements detected for
  package DiffEqDevTools [f3b72e0c]: restricted by compatibility
  requirements with RecursiveArrayTools to versions: uninstalled``. The
  local lib/DiffEqDevTools already declares RAT 4 and SciMLBase 3, so
  path-sourcing it unblocks the resolve on all four DelayDiffEq test
  groups (Integrators / Interface / QA / Regression).

Scope intentionally narrow: other sublibraries that list DiffEqDevTools
in [extras] without [sources] still pass CI because their test envs
don't pull in a path package that forces RAT 4; they can be migrated
later as the registered SciMLBase-v3 DiffEqDevTools lands.

Co-authored-by: ChrisRackauckas-Claude <accounts@chrisrackauckas.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

lib/DelayDiffEq/Project.toml

@@ -54,6 +54,7 @@ StochasticDiffEqImplicit = {path = "../StochasticDiffEqImplicit"}
 StochasticDiffEqLowOrder = {path = "../StochasticDiffEqLowOrder"}
 StochasticDiffEqROCK = {path = "../StochasticDiffEqROCK"}
 DiffEqBase = {path = "../DiffEqBase"}
+DiffEqDevTools = {path = "../DiffEqDevTools"}
 
 [compat]
 ADTypes = "1.16"

test/regression/ode_dense_tests.jl

@@ -111,7 +111,11 @@ function regression_test(
     sol(interpolation_results_2d, interpolation_points)
     sol(interpolation_points[1])
     sol2 = solve(prob_ode_2Dlinear, alg, dt = 1 // 2^(4), dense = true, adaptive = false)
-    for i in eachindex(sol2)
+    # `eachindex(sol2)` now returns `CartesianIndices` over the full solution
+    # tensor (u_dims..., nsteps) under RecursiveArrayTools v4; iterate the
+    # timestep axis via `sol2.u` so `sol2.u[i]` / `interpolation_results_2d[i]`
+    # index the per-step matrix as the test intends.
+    for i in eachindex(sol2.u)
         print_results(
             @test maximum(maximum.(abs.(sol2.u[i] - interpolation_results_2d[i]))) <
                 tol_ode_2Dlinear