Interpolating Adjoint possible test fix

[jClugstor]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

I was getting some test failures in the adjoint tests when testing SciML/OrdinaryDiffEq.jl#2567
https://github.com/SciML/OrdinaryDiffEq.jl/actions/runs/13182182796/job/36795499058?pr=2567

It looks like in one of the interpolating adjoint tests, the value of t was going slightly above the highest value of the last interval. This was causing findcursor(intervals,t) to return one higher than the length of intervals, so there would be an error when trying to use that as an index to intervals. This fixes the issue, but I'm not sure that it's the correct thing to do, given that probably t should never be higher than the highest value of the last interval in the first place.

function split_states(du, u, t, S::TS;
        update = true) where {TS <:
                              ODEInterpolatingAdjointSensitivityFunction}
    (; sol, y, checkpoint_sol, discrete, prob, f) = S
    idx = length(y)
    if update
        if checkpoint_sol === nothing
            if t isa ForwardDiff.Dual && eltype(S.y) <: AbstractFloat
                y = sol(t, continuity = :right)
            else
                sol(y, t, continuity = :right)
            end
        else
            intervals = checkpoint_sol.intervals
            interval = intervals[checkpoint_sol.cursor]
            if !(interval[1] <= t <= interval[2])
                cursor′ = findcursor(intervals, t)
                cursor′ = cursor′ > length(intervals) ? length(intervals) : cursor′
                interval = intervals[cursor′]
                cpsol_t = current_time(checkpoint_sol.cpsol)
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