Remove Tracker @test_broken guards (closes #1331)

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas.

Depends on SciML/OrdinaryDiffEq.jl#3663 (DiffEqBase v7.5.1). This PR will fail CI until that PR is merged and a new DiffEqBase patch is tagged.

Closes #1331.

Summary

• Drops the five if backend_name == "Tracker" && VERSION >= v"1.12"; @test_broken false; continue; end guards in test/concrete_solve_derivatives.jl (save_idxs, save_everystep=false, non-integer saveat=2.3, VecOfArray, BouncingBall).
• Bumps DiffEqBase compat to 7.5.1.

The issue was filed as "Tracker fails on Julia 1.12+", but the actual trigger is the RecursiveArrayTools v4 supertype change (AbstractVectorOfArray <: AbstractArray). On RAT v3, the Tracker @grad for solve_up fell through to convert(AbstractArray, sol) which stack'd sol.u into a flat matrix. On RAT v4, the new sol isa AbstractArray branch ships the nested Vector{Vector{Float64}} to Tracker, and sum(solve(...)) reduces element-wise into a vector, tripping Tracker.gradient's "Function output is not scalar" check. Full diagnosis on #1331.

The fix lives in DiffEqBase (SciML/OrdinaryDiffEq.jl#3663); this PR is the test-side cleanup confirming the previously-broken tests now pass.

Verification

With the DiffEqBase patch applied locally on Julia 1.12.6 / Tracker 0.2.38 / RAT 4.3.0:

Test Summary: | Pass  Total     Time
save_idxs     |    1      1  1m14.8s
Test Summary:        | Pass  Total   Time
save_everystep=false |    1      1  11.6s
Test Summary: | Pass  Total   Time
saveat=2.3    |    1      1  23.9s
Test Summary: | Pass  Total   Time
VecOfArray    |    1      1  14.7s
Test Summary: | Pass  Total   Time
BouncingBall  |    1      1  39.4s

Test plan

• Land Fix Tracker gradient through solve on RecursiveArrayTools v4 OrdinaryDiffEq.jl#3663 and tag DiffEqBase v7.5.1.
• Once DiffEqBase 7.5.1 is registered, re-run CI on this PR; the five previously-skipped Tracker testsets should pass without @test_broken.

[ChrisRackauckas-Claude]

Local verification on Julia 1.12.6 + this PR branch (cc/remove-tracker-broken-guards-1331) shows the released DiffEqBase 7.5.1 does not actually fix #1331 — so this PR will still fail CI even on the new 7.5.1 release.

What I found

PR SciML/OrdinaryDiffEq.jl#3665 (merged 2026-05-21T13:48Z, released as DiffEqBase 7.5.1 at 14:08Z) was supposed to add a new if sol isa RecursiveArrayTools.AbstractVectorOfArray; return sol, pb_f; end branch before the existing if sol isa AbstractArray check in DiffEqBaseTrackerExt.jl. The original commit on that PR (61b0c61) included that branch. A follow-up commit on the same PR (139911b, "Update DiffEqBaseTrackerExt.jl") deleted it before the merge, leaving only an effectively-unreachable fallback change:

- return convert(AbstractArray, sol), pb_f
+ return sol, pb_f

Under RAT v4, AbstractVectorOfArray <: AbstractArray, so the existing if sol isa AbstractArray branch matches ODESolution first and still returns sol.u :: Vector{Vector{Float64}} to Tracker — the original bug. The fallback is never hit for ODESolution.

Reproduction with the released 7.5.1

Smoke MWE (Tracker.gradient(p -> sum(solve(remake(prob; p), Tsit5(); kw...)), p0)):

FAIL  NamedTuple()                => Function output is not scalar
FAIL  (save_idxs = [1],)          => Function output is not scalar
FAIL  (save_everystep = false,)   => Function output is not scalar
FAIL  (saveat = 2.3,)             => Function output is not scalar

This PR's first testset (save_idxs Tests → save_idxs - Tracker) on Julia 1.12.6 / DiffEqBase 7.5.1 (released):

save_idxs - Tracker: Error During Test
  Got exception outside of a @test
  Function output is not scalar
  [...]
    [2] losscheck(x::Tracker.TrackedVector{Float64, Vector{Float64}})
      @ Tracker ~/.julia/packages/Tracker/dLAzA/src/back.jl:153

Proposed path forward

I've opened SciML/OrdinaryDiffEq.jl#3666, which restores the dropped if sol isa AbstractVectorOfArray; return sol, pb_f; end branch and bumps DiffEqBase to 7.5.2. Verified locally on Julia 1.12.6 with that patch dev'd in:

• Smoke MWE: 4/4 PASS, all produce scalar tracked gradients
• The 5 previously-@test_broken testsets in this PR (via a focused runner that replicates the prelude + the 5 testsets verbatim):

  Test Summary:              | Pass  Total     Time
  save_idxs Tests            |    2      2  1m24.3s
  save_everystep=false Tests |    2      2  16.4s
  Non-integer saveat Tests   |    2      2  28.2s
  VecOfArray Derivatives     |    2      2  16.2s
  BouncingBall ODE           |    3      3  45.7s
  

So:

1. Land Restore AbstractVectorOfArray branch in Tracker @grad (RAT v4) OrdinaryDiffEq.jl#3666 and tag DiffEqBase 7.5.2.
2. Bump this PR's DiffEqBase compat from 7.5.1 → 7.5.2.
3. Re-run CI here; the five testsets should pass.

Recommend holding merge of this PR until 7.5.2 is released. /cc @ChrisRackauckas