CG4a convergence order test failing in OrdinaryDiffEqLinear

[singhharsh1708]

Bug

The CG4a convergence order test in OrdinaryDiffEqLinear is failing on both master and the current fix branches.

Failing test (linear_method_tests.jl:81):

A = MatrixOperator(ones(2, 2), update_func! = update_func!)
prob = ODEProblem(A, ones(2), (0, 30.0))
dts = (0.38) .^ (6:-1:1)
test_setup = Dict(:alg => Vern9(), :reltol => 1.0e-14, :abstol => 1.0e-14)
sim = analyticless_test_convergence(dts, prob, CG4a(), test_setup)
@test sim.𝒪est[:l2] ≈ 4 atol = 0.28  # FAILS

where update_func! is:

function update_func!(A, u, p, t)
    A[1, 1] = 0
    A[2, 1] = sin(u[1])
    A[1, 2] = -1
    A[2, 2] = 0
end

Expected

CG4a is a 4th order method — the estimated convergence order should be ≈ 4 within atol = 0.28.

Observed

Test fails, suggesting the measured convergence order is outside the [3.72, 4.28] range for this problem/dt grid combination.

Notes

• Confirmed pre-existing on master (not introduced by any recent PR)
• The tolerance atol = 0.28 is already looser than the other CG methods in the same file
• May need a different dts grid or tighter reference solver tolerance for this nonlinear problem

[ChrisRackauckas]

Bisect to where this was introduced?