Error when adding/subtracting `TensorMap` and `DiagonalTensorMap`

[lkdvos]

Currently, in VectorInterface.add we are not correctly handling the case where one of the tensors is diagonal, but the other is not:

using TensorKit
t = rand(ComplexSpace(2), ComplexSpace(2))
d = DiagonalTensorMap(rand(2), ComplexSpace(2))

t - t # fine
d - d # fine
t - d # fine
d - t # error:

julia> d - t
ERROR: ArgumentError: cannot set off-diagonal entry (2, 1) to a nonzero value (-0.09276549337779227)
Stacktrace:
  [1] setindex!
    @ ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/diagonal.jl:226 [inlined]
  [2] _setindex!
    @ ./abstractarray.jl:1473 [inlined]
  [3] setindex!
    @ ./abstractarray.jl:1443 [inlined]
  [4] macro expansion
    @ ./broadcast.jl:995 [inlined]
  [5] macro expansion
    @ ./simdloop.jl:77 [inlined]
  [6] copyto!
    @ ./broadcast.jl:994 [inlined]
  [7] copyto!(dest::LinearAlgebra.Diagonal{…}, bc::Base.Broadcast.Broadcasted{…})
    @ LinearAlgebra ~/.julia/juliaup/julia-1.12.4+0.x64.linux.gnu/share/julia/stdlib/v1.12/LinearAlgebra/src/structuredbroadcast.jl:207
  [8] materialize!
    @ ./broadcast.jl:905 [inlined]
  [9] materialize!
    @ ./broadcast.jl:902 [inlined]
 [10] add!(y::LinearAlgebra.Diagonal{…}, x::Base.ReshapedArray{…}, α::Float64, β::VectorInterface.One)
    @ VectorInterface ~/.julia/packages/VectorInterface/J6qCR/src/abstractarray.jl:89
 [11] add!(ty::DiagonalTensorMap{…}, tx::TensorMap{…}, α::Float64, β::VectorInterface.One)
    @ TensorKit /home/ldevos/LocalProjects/TensorKit.jl/src/tensors/vectorinterface.jl:79
 [12] add!
    @ ~/.julia/packages/VectorInterface/J6qCR/src/interface.jl:124 [inlined]
 [13] add(ty::DiagonalTensorMap{…}, tx::TensorMap{…}, α::Float64, β::VectorInterface.One)
    @ TensorKit /home/ldevos/LocalProjects/TensorKit.jl/src/tensors/vectorinterface.jl:72
 [14] add
    @ ~/.julia/packages/VectorInterface/J6qCR/src/interface.jl:108 [inlined]
 [15] -(t1::DiagonalTensorMap{Float64, ComplexSpace, Vector{…}}, t2::TensorMap{Float64, ComplexSpace, 1, 1, Vector{…}})
    @ TensorKit /home/ldevos/LocalProjects/TensorKit.jl/src/tensors/linalg.jl:9