Could not simultaneously block-diagonalize

On fd7336e, I am hitting an error on line 189 of wedderburn.jl

Firstly, the actual error raised is "ERROR: LoadError: UndefVarError: `dx` not defined" because of a minor bug on line 190, but the real underlying problem is that the simultaneous block-diagonalization fails. Unfortunately the example (attached) I have to repro this is rather resource intensive and it seems to run fine without an error at lower polynomial degree (though I have not exhaustively tested).

As with #381, I create a symmetry group with Oscar, a basis invariant under this group, a polynomial using that basis and then an SOS constraint from that polynomial.

[sos_repro.jl.txt](https://github.com/user-attachments/files/17475296/sos_repro.jl.txt)

Start with wedderburn.jl around lines 189-190 and run the attached sos_repro.jl.txt example on fd7336e. Investigate the reported dx error separately from the simultaneous block-diagonalization failure at the higher polynomial degree. Done means the resource-intensive reproduction no longer fails in the reported way.

Written by the indexing model from the issue text.