Constraint on variable overwritten

[laradicp]

Describe the bug
When adding a new constraint on variable, it may overwrite previous constraints of the same type.

To Reproduce

# x >= 1
c1 = MOI.add_constraint(
    model,
    MOI.SingleVariable(x),
    MOI.GreaterThan(1),
)
# x >= 2
c2 = MOI.add_constraint(
    model,
    MOI.SingleVariable(x),
    MOI.GreaterThan(2),
)

Impossible to delete c2 without losing c1 because Coluna._constraint_on_variable! called by MOI.add_constraint changes the perennial bounds of the variables.

Additional context
Maybe related to the incomplete dual solution (missing MOI.ConstraintDual for SingleVariable constraints) and to #531.

[guimarqu]

Coluna will now throw an error MOI.LowerBoundAlreadySet. (see #628)