MOI.is_valid can be falsely positive for bridged constraints

Found as part of #2695. But it does't need to hold that up merging.

The last `MOI.is_valid` should return `false`:

```Julia
julia> import MathOptInterface as MOI

julia> include("test/Bridges/sdpa_models.jl")

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

julia> x = MOI.add_variables(model, 2);

julia> c = MOI.add_constraint(model, MOI.VectorOfVariables(x), MOI.Nonpositives(2));

julia> MOI.is_valid(model, c)
true

julia> MOI.is_valid(model, typeof(c)(c.value + 1))
true
```

See
https://github.com/jump-dev/MathOptInterface.jl/pull/2695/files#diff-87c703874f7251d45788070ce6ab50219cfe8b7561d08e6f6148d44afba24ccbR249-R251

Start with the reproduction in test/Bridges/sdpa_models.jl and verify the two MOI.is_valid calls for the bridged constraint. Then inspect the referenced pull request diff, especially the linked lines, to trace how constraint validity is checked. Done means the original constraint remains valid while the incremented index returns false.

Rédigé par le modèle d'indexation à partir du texte de l'issue.