DIT errors with custom types in contexts

Start at DifferentiationInterfaceTest/src/scenarios/scenario.jl:78, where zero_contexts handles DI.Constant, and reproduce the shown DIT.Scenario call with MyStruct. Trace how context zero values are constructed and add regression coverage for this custom context; done when the example scenario runs without the shown zero(::MyStruct) error.

import DifferentiationInterface as DI
import DifferentiationInterfaceTest as DIT

struct MyStruct{V}
    v::V
end
ms = MyStruct(randn(2))
f(x, ms) = sum(x .* ms.v)

using ForwardDiff
DI.gradient(f, DI.AutoForwardDiff(), randn(2), DI.Constant(ms)) # This is fine

DIT.Scenario{:gradient,:out}(f, randn(2), DI.Constant(ms)) # This errors

Error:

julia> DIT.Scenario{:gradient,:out}(f, randn(2), DI.Constant(ms))
ERROR: MethodError: no method matching zero(::MyStruct{Vector{Float64}})
The function `zero` exists, but no method is defined for this combination of argument types.

Closest candidates are:
  zero(::Type{Union{}}, Any...)
   @ Base number.jl:359
  zero(::Type{LibGit2.GitHash})
   @ LibGit2 ~/.julia/juliaup/julia-1.13.0+0.aarch64.apple.darwin14/Julia-1.13.app/Contents/Resources/julia/share/julia/stdlib/v1.13/LibGit2/src/oid.jl:221
  zero(::Type{Pkg.Resolve.VersionWeight})
   @ Pkg ~/.julia/juliaup/julia-1.13.0+0.aarch64.apple.darwin14/Julia-1.13.app/Contents/Resources/julia/share/julia/stdlib/v1.13/Pkg/src/Resolve/versionweights.jl:15
  ...

Stacktrace:
 [1] call_composed
   @ ./operators.jl:1110 [inlined]
 [2] (::ComposedFunction{…})(x::DifferentiationInterface.Constant{…}; kw::@Kwargs{})
   @ Base ./operators.jl:1107
 [3] map
   @ ./tuple.jl:354 [inlined]
 [4] zero_contexts(contexts::DifferentiationInterface.Constant{MyStruct{Vector{Float64}}})
   @ DifferentiationInterfaceTest ~/jl/DifferentiationInterface.jl/DifferentiationInterfaceTest/src/scenarios/scenario.jl:78
 [5] (DifferentiationInterfaceTest.Scenario{…})(f::Function, x::Vector{…}, contexts::DifferentiationInterface.Constant{…})
   @ DifferentiationInterfaceTest ~/jl/DifferentiationInterface.jl/DifferentiationInterfaceTest/src/scenarios/scenario.jl:81
 [6] top-level scope
   @ REPL[19]:1
 [7] top-level scope
   @ REPL:1
Some type information was truncated. Use `show(err)` to see complete types.