Re-enable Enzyme AD tests in FIRK after upstream fix

[ChrisRackauckas-Claude]

Context

Enzyme tests in lib/BoundaryValueDiffEqFIRK/test/expanded/ad_tests.jl were removed in #424 because Enzyme BatchDuplicated forward mode crashes on closures capturing Vector{Vector{Float64}}, which is the pattern used by the FIRK solver.

The upstream bug is tracked at EnzymeAD/Enzyme.jl#2936.

Action needed

When the upstream Enzyme bug is fixed, re-add the Enzyme test variants:

using Enzyme

# For multipoint BVP tests:
jac_alg_enzyme = BVPJacobianAlgorithm(
    bc_diffmode = AutoSparse(
        AutoEnzyme(mode = Enzyme.Reverse, function_annotation = Enzyme.Duplicated)
    ),
    nonbc_diffmode = AutoEnzyme(mode = Enzyme.Forward, function_annotation = Enzyme.Duplicated)
)

# For twopoint BVP tests:
jac_alg_enzyme = BVPJacobianAlgorithm(
    AutoSparse(
        AutoEnzyme(mode = Enzyme.Forward, function_annotation = Enzyme.Duplicated)
    )
)

Also restore AutoEnzyme(mode = Enzyme.Forward, ...) as the nonbc_diffmode in the Mooncake test variants.

[ChrisRackauckas-Claude]

Update: The original BatchDuplicated crash (#2936) was fixed in Enzyme 0.13.123, but there's a second bug that still affects the FIRK solver.

A new minimal MWE has been filed at EnzymeAD/Enzyme.jl#2940 - this triggers when a closure captures a callable struct with Vector{Vector{Float64}} combined with similar(vector) + similar(matrix) + mul! in a loop.

Both upstream issues need to be fixed before Enzyme can be re-enabled in the FIRK AD tests.