TuringLang / JuliaBUGS.jl / 14757075030

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Ensure only model parameters are included in the sorted parameter list (#292)

If a Julia code to compute log density can be generated for a model,
then the topo order of nodes in the model need to be updated to sync
with the generated Julia source.

The original code used the topologically sorted list of all nodes
(`pass.sorted_nodes`) to determine the final ordered list of parameters
for the `Model`. However, `pass.sorted_nodes` can include variables that
are purely "transformed data" - deterministic nodes computed entirely
from provided data constants at compile time (as described in
`docs/src/source_gen.md` under "Handling Mixed Data Transformation and
Deterministic Assignments"). For example, if `y = [1, 2, missing,
missing, 2]` and the model has `x[i] = y[i] + 1`, then `x[1]`, `x[2]`,
and `x[5]` are transformed data, while only `x[3]` and `x[4]` (dependent
on the missing `y` values) are actual deterministic parameters within
the model graph. Including transformed data nodes in the final
`parameters` list is incorrect, as they are constants and not part of
the model's parameter space to be evaluated or sampled.

This PR is a simple fix by filtering the `pass.sorted_nodes` list,
keeping only those `VarName`s that are already present in the
`parameters` set. This ensures the final `parameters` field of the
`Model` struct contains only true model parameters, correctly ordered
according to the graph's topology.

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