fix: random circuit codes are non-CSS so return iscss(...) = false instead of nothing

[Fe-r-oz]

Hi, @royess, The random_circuit codes are non-CSS so, I am wondering why the iscss(::Type{...}) returns nothing. Shouldn't it return false? Hopefully, that is not a bad proposition?

Could you explain if it was a typo or there is some reason behind it? Thanks! I agree with you that codes are non-CSS.

BP and BP-OSD in PyQDecoders.jl (PyMatchingDecoder ) are not applicable to random circuit codes because they are non-CSS.

QuantumClifford.jl/src/ecc/codes/random_circuit.jl

Line 19 in 0d13791

iscss(::Type{CircuitCode}) = nothing

Bravyi introduces a Mixed Integer Programming for CSS/QLDPC codes so his approach don't work for non-CSS codes. Hence, I want to exclude non-CSS codes so user does not end up calculating distance for non-CSS codes which throw solver error.

iscss(::Type{...}) = false would be helpful here because in a later PR, I want to do something like

function distance(c::AbstractECC; kwargs...)
    if !iscss(c)
        throw(ArgumentError("Computing the minimum distance of QLDPC using Mixed Integer Programming works only for CSS codes."))
    end
    return distance(parity_checks(c); kwargs...)
end

@Krastanov, Please help review this PR, Thank you!

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