fix: propagate __options__ to child Context for multi-/named-output submodels

[docxology]

Description

Fixes #1. When a composite submodel is called with a multi-output ((a, b) ~ sub(...)) or
named-output ((a = x, b = y) ~ sub(...)) left-hand side, the generated make_node!
built the child Context without the NodeCreationOptions from the where { ... } clause.
The single-output path already passes it. As a result inline where { constraints = ... }
(and any compound-context option) was silently ignored for those submodels.

Change

src/model_macro.jl — in both generated composite make_node! bodies:

- __context__ = GraphPPL.Context(__parent_context__, $ms_name)
+ __context__ = GraphPPL.Context(__parent_context__, $ms_name, __options__)

(Tuple-LHS path, previously line ~860; NamedTuple-LHS path, previously line ~882.)

Tests

Added a regression test in test/plugins/variational_constraints/variational_constraints_tests.jl
asserting that a multi-output submodel call with where { constraints = ... } exposes the
expected Constraints via get(context_options(inner_context), :constraints, nothing) and
materializes the requested factorization — mirroring the existing single-output
"inline constraints on submodel calls" test.

Verification

• test/plugins/variational_constraints/variational_constraints_tests.jl: pass
• test/multi_and_zero_output_tests.jl: pass