Integrators_II resize test calls length on a DiffCache

[ChrisRackauckas-Claude]

Reproduction

On an isolated detached worktree at unmodified OrdinaryDiffEq master commit 4cb802090d69e83e13a43c2490d9de40bb3216bc, Julia 1.10.11, the canonical group command fails:

GROUP=Integrators_II ~/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'

The preceding Integrators_II subgroups pass. Resize Tests then reports:

resize! with SplitODEProblem: Error During Test
Expression: length(integrator.f._func_cache) == 2
MethodError: no method matching length(
    ::PreallocationTools.DiffCache{Vector{Float64}, Vector{Float64}}
)

Test Summary:                  | Pass  Error  Total
Resize Tests                   |   89      1     90
  resize! with SplitODEProblem |    5      1      6

A focused reproduction on the same checkout is:

using OrdinaryDiffEq

f!(du, u, p, t) = du .= u
ode = SplitODEProblem(f!, f!, [1.0], (0.0, 1.0))
integrator = init(ode, Tsit5())
step!(integrator)
resize!(integrator, 2)

@show typeof(integrator.f._func_cache)
length(integrator.f._func_cache)

It prints:

PreallocationTools.DiffCache{Vector{Float64}, Vector{Float64}}
ERROR: MethodError: no method matching length(
    ::PreallocationTools.DiffCache{Vector{Float64}, Vector{Float64}}
)

The same failure occurs in the Julia 1.12.6 CI job:
https://github.com/SciML/OrdinaryDiffEq.jl/actions/runs/31294134378/job/93196369385

Introduction point

The invalid assertion was added by:
6bc9d9a

Its parent does not contain length(integrator.f._func_cache); the new line is therefore the direct introduction point for this deterministic error. _func_cache is a PreallocationTools.DiffCache, which deliberately is not an AbstractArray and has no Base.length method.

Expected fix

Keep the deterministic post-resize state initialization, but validate scratch-buffer growth through behavior or a supported cache accessor rather than calling length on the DiffCache wrapper. The test also currently emits the independent deprecation warning that u_modified!(integrator, true) should use derivative_discontinuity!(integrator, true).