PR draft — Validate captured value in ClosedForm closure extraction (fixes issue-06)

[docxology]

Problem

exmplate preprocess_strategy_argument(::ClosedFormStrategy, argument::F) takes getfield(argument, first(fieldnames(F))) with no type check; multi-variable closures silently retrieve the wrong (first) captured value.

Change

ext/ClosedFormExpectationsExt/ClosedFormExpectationsExt.jl:

-    captured = getfield(argument, first(field_names))
-    return (strategy, Logpdf(captured))
+    captured = getfield(argument, first(field_names))
+    if !(captured isa Union{Distribution,ProductOf})
+        error(
+            "`ClosedFormStrategy` expected the first captured variable of the closure to be a " *
+            "`Distribution` or `ProductOf`, but got `$(typeof(captured))`. `ClosedFormStrategy` " *
+            "supports closures that capture exactly one distribution. " *
+            "Pass the `Distribution`/`ProductOf` directly to avoid ambiguity.",
+        )
+    end
+    return (strategy, Logpdf(captured))

Optionally, when more than one field is captured, prefer the field whose value is a Distribution/ProductOf and error only if none is found.

Verification

• let a = 5, d = Normal(0,1); fn=(x)->logpdf(d,x)+a; preprocess_strategy_argument(...) now errors with a clear message instead of silently projecting the scalar 5.
• Single-distribution closures still work.
• Full suite green.