Address Copilot round 3: reuse extract_iter_variable_name for list comprehensions

- Refactor build_list_comprehension_node() to reuse the shared
  extract_iter_variable_name() helper, so `var IN list` validation
  is consistent between list comprehensions and predicate functions
  (all/any/none/single). Qualified ColumnRefs like `x.y IN list`
  are now rejected in list comprehensions the same way they are
  in predicate functions.
- Update list_comprehension expected output for the normalized
  lowercase "syntax error at or near IN" message.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: gregfelice

regress/expected/list_comprehension.out

@@ -577,11 +577,11 @@ LINE 1: ...$$ RETURN [i IN range(0, 10, 2) WHERE i>5 | i^2], i $$) AS (...
 HINT:  variable i does not exist within scope of usage
 -- Invalid list comprehension
 SELECT * FROM cypher('list_comprehension', $$ RETURN [1 IN range(0, 10, 2) WHERE 2>5] $$) AS (result agtype);
-ERROR:  Syntax error at or near IN
+ERROR:  syntax error at or near IN
 LINE 1: SELECT * FROM cypher('list_comprehension', $$ RETURN [1 IN r...
                                                     ^
 SELECT * FROM cypher('list_comprehension', $$ RETURN [1 IN range(0, 10, 2) | 1] $$) AS (result agtype);
-ERROR:  Syntax error at or near IN
+ERROR:  syntax error at or near IN
 LINE 1: SELECT * FROM cypher('list_comprehension', $$ RETURN [1 IN r...
                                                     ^
 -- Issue - error using list comprehension with WITH *

src/backend/parser/cypher_gram.y

@@ -3339,29 +3339,22 @@ static Node *build_list_comprehension_node(Node *var, Node *expr,
                                            int location)
 {
     SubLink *sub;
-    String *val;
-    ColumnRef *cref = NULL;
+    ColumnRef *cref;
+    char *varname;
     cypher_list_comprehension *list_comp = NULL;
 
-    if (!IsA(var, ColumnRef))
-    {
-        ereport(ERROR,
-                (errcode(ERRCODE_SYNTAX_ERROR),
-                 errmsg("Syntax error at or near IN")));
-    }
-
-    cref = (ColumnRef *)var;
-    val = linitial(cref->fields);
-    if (!IsA(val, String))
-    {
-        ereport(ERROR,
-                (errcode(ERRCODE_SYNTAX_ERROR),
-                 errmsg("Invalid list comprehension variable name")));
-    }
+    /*
+     * Reuse the shared iterator-name helper so list comprehensions and
+     * predicate functions (all/any/none/single) validate `var IN list`
+     * the same way — in particular, qualified ColumnRefs like `x.y` are
+     * rejected rather than silently treated as iterator `x`.
+     */
+    varname = extract_iter_variable_name(var);
+    cref = (ColumnRef *) var;
 
     /* build the list comprehension node */
     list_comp = make_ag_node(cypher_list_comprehension);
-    list_comp->varname = val->sval;
+    list_comp->varname = varname;
     list_comp->expr = expr;
     list_comp->where = where;
     list_comp->mapping_expr = (mapping_expr != NULL) ? mapping_expr :