Zygote DAE adjoint test errors with ChainRules PConstructorApplicator issue

## Summary The AutoZygote test in `test/downstream/observables_autodiff.jl` (line 227, "Adjoints with DAE" testset) errors with: ``` Tuple field type cannot be Union{} ``` at `ChainRules.may_bc_derivatives(::Type{Union{}}, f::ModelingToolkitBase.PConstructorApplicator{typeof(identity)}, args::Tuple{})`. ## Details This affects Julia < 1.12 (where Zygote backends are used). The error occurs in ChainRules' broadcast derivatives handling when Zygote tries to differentiate through a `ModelingToolkitBase.PConstructorApplicator`. This test was previously masked because the Downstream test group failed earlier at `ensemble_diffeq.jl:21` (fixed in #1231). Once that was fixed, the test group now reaches the `observables_autodiff.jl` tests and hits this ChainRules/Zygote issue. ## Workaround The test has been marked as `@test_broken` in #1231. ## Root Cause This is a downstream issue in ChainRules' broadcast differentiation rules interacting with ModelingToolkitBase's `PConstructorApplicator` type.