AST visitors silently drop `VARIABLE`-typed children — affects `FunctionInvocation::arguments()`, `IdInColl::collection()`, `ForeachClause::list()`

[arvindsh]

Summary

Three sibling AST visitors filter out SyntaxKind::VARIABLE children
across the board when they should only skip the specific VARIABLE
node that is the iteration variable / function name. As a result, any
expression that uses a variable as a function argument or collection
silently loses that argument and surfaces downstream as either:

• "<fn>() takes exactly N arguments" (the function appears to be
  called with the wrong arity),
• "missing collection in list comp" (the comprehension's collection
  appears to be missing), or
• a silent zero-result for FOREACH over a variable-typed list.

Affected sites

Location	Symptom
src/syntax/ast/expressions.rs::FunctionInvocation::arguments()	Every variable-reference function argument is dropped. abs(x), labels(n), size(s), head(lst), nodes(p), relationships(p), jaccard(s1, s2) all appear to be called with zero args. Literal arguments (abs(42), size([1,2,3])) survive because they are not VARIABLE nodes.
src/syntax/ast/expressions.rs::IdInColl::collection()	The collection in a list comprehension or IN-binding is dropped when it is a bare variable reference. [x IN s1 WHERE …] fails with internal error: missing collection in list comp. [x IN [1,2,3] WHERE …] works because the literal is not a VARIABLE.
src/syntax/ast/clauses.rs::ForeachClause::list()	The list expression in FOREACH (var IN someList…) is dropped when someList is a variable reference.

Reproducer

WITH 42 AS s1 RETURN abs(s1)            -- "abs() takes exactly 1 argument"
WITH [1,2,3] AS s RETURN size(s)        -- "size() takes exactly 1 argument"
WITH [1,2] AS a, [2,3] AS b
RETURN jaccard(a, b)                    -- "jaccard() takes exactly 2 arguments"

WITH [1,2,3] AS s1, [3,4,5] AS s2
RETURN [x IN s1 WHERE x IN s2]          -- "missing collection in list comp"

MATCH (n) WITH collect(n) AS ns FOREACH (m IN ns | DETACH DELETE m)
                                        -- silently does nothing on the variable-typed list