[Bridges] fix deleting variable with constraint bridges

[odow]

Closes #2817

I don't think this is the proper fix. But passes the test. I want to run solver-tests with it if CI is green

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/17026926756

[odow]

The underlying cause is the same as #2696

We don't really have a chain-of-custody to discriminate constraints that were added by the user at the top level from constraints added by bridges, unless we walk all the bridges to check.

julia> MOI.Utilities.@model(
           Model2817b,
           (),
           (MOI.LessThan,),
           (),
           (),
           (),
           (MOI.ScalarAffineFunction,),
           (),
           ()
       );

julia> function MOI.supports_constraint(
           ::Model2817b,
           ::Type{MOI.VariableIndex},
           ::Type{S},
       ) where {S<:MOI.AbstractScalarSet}
           return false
       end

julia> model = MOI.instantiate(Model2817b{Float64}; with_bridge_type = Float64);

julia> y, _ = MOI.add_constrained_variables(model, MOI.Nonpositives(1));

julia> ci = MOI.ConstraintIndex{MOI.VariableIndex,MOI.LessThan{Float64}}(1)
MathOptInterface.ConstraintIndex{MathOptInterface.VariableIndex, MathOptInterface.LessThan{Float64}}(1)

julia> MOI.is_valid(model, ci)
true

[odow]

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/17029443959

[odow]

Okay. I think that's everything passing tests at least. Now to document what exactly I did...

[odow]

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/17031397524

[blegat]

What do you mean by different bridge ? It seems to me that if a bridge tries to delete a variable, now it won't delete any constraint created on that variable. So we are kind of assuming that the bridges can't rely on the fact that when they delete variables, the constraint on this variable won't be delete automatically, while the user can expect this behavior

[odow]

Hmm. Maybe I need to check this. I thought if a bridge deletes a variable it will, by the logic in this function, delete the related constraints.

[blegat]

I think we should also clarify in the docstring of delete for bridges that they should delete all constraints by themself and not rely on the automatic deletion of variable-constraints that are deleted by the variables.

[odow]

for bridges that they should delete all constraints by themself and not rely on the automatic deletion of variable-constraints that are deleted by the variables

I don't think this is necessary. Do you have an example that fails? If a bridge has, for example, added a binary variable, the ZeroOne constraint can't exist if the variable has been deleted.

[odow]

https://github.com/jump-dev/MathOptInterface.jl/actions/runs/17059980958

[odow]

Confirmed that it's not necessary, and I've added a test for it.