`@warn` in apply_constraints! throws `UndefVarError: opt` when a node already has a form constraint

[docxology]

Summary

In src/plugins/variational_constraints/variational_constraints_engine.jl, two defensive
branches meant to warn "node already has a functional-form constraint, skipping" instead
raise UndefVarError: opt not defined in GraphPPL because the lazy"..." message
interpolates undefined local variables opt and constraint_data.

Locations

• Line 954 — apply_constraints! for MarginalFormConstraint{…IndexedVariable…}:
  @warn lazy"Node $node already has functional form constraint $(opt[:q]) applied, therefore $constraint_data will not be applied"
• Line 969 — apply_constraints! for MessageFormConstraint:
  same message.

Neither function scope binds opt or constraint_data.

Impact

The branch exists to give users a clear signal that a repeated form constraint is being
skipped. Instead of that warning, the user sees a confusing error record
("Exception while generating log record … UndefVarError: opt not defined"). The skip itself
still executes, so inference is not corrupted — this is a broken diagnostic.

Repro

using GraphPPL, Distributions
import GraphPPL: @model
@model function m(xs)
    x ~ Normal(0.0, 1.0)
    xs[1] ~ Normal(x, 1.0)
end
model = GraphPPL.create_model(m()) do m, ctx
    xs = GraphPPL.datalabel(m, ctx, GraphPPL.NodeCreationOptions(kind=:data), :xs, [1.0, 2.0])
    return (xs = xs,)
end
ctx = GraphPPL.getcontext(model)
xnode = first(collect(filter(GraphPPL.as_variable(:x), model)))
GraphPPL.setextra!(model[xnode], GraphPPL.VariationalConstraintsMarginalFormConstraintKey, :some_form)
GraphPPL.apply_constraints!(model, ctx, GraphPPL.MarginalFormConstraint(GraphPPL.IndexedVariable(:x, nothing), Normal))
# => UndefVarError: opt not defined in GraphPPL  (at variational_constraints_engine.jl:954)

Suggested fix

Use bound locals, e.g.:

@warn "Node $node already has functional form constraint $(getconstraint(marginal_constraint)) applied, therefore it will not be applied"

and the analogous MessageFormConstraint variant. Add a regression test that the warning
fires and the original constraint is preserved.