SCCNonlinearProblem construction fails with SymbolicUtils.DefaultSubstituter API mismatch

## Description The `SCCNonlinearProblem` construction from a ModelingToolkit `System` fails with a `MethodError` from `SymbolicUtils.DefaultSubstituter`. ## Error ``` MethodError: no method matching (SymbolicUtils.DefaultSubstituter{true})(::ModelingToolkitBase.MTKParameters{...}, ::Symbolics.FPSubFilterer{...}) ``` The error occurs in `ModelingToolkitBase/src/problems/linearproblem.jl:98` during `get_A_b_from_LinearFunction`, which calls into SymbolicUtils substitution with an `MTKParameters` object where an `AbstractDict` is expected. ## Affected test `test/downstream/problem_interface.jl` - the `@testset "SCCNonlinearProblem"` block. This test has been temporarily marked as `@test_broken`. ## Root cause This appears to be an API mismatch between the current versions of SymbolicUtils and ModelingToolkitBase. The `DefaultSubstituter` constructor expects different argument types than what ModelingToolkit is providing. ## Action needed This likely needs a fix in ModelingToolkit or SymbolicUtils to align the substituter API. Once fixed, the `@test_broken` annotation in `problem_interface.jl` should be reverted back to the full test.