Fix spaced bar syntax in codes mode

The spaced bar syntax `| |` failed with syntax_error(incomplete_reduction)
when double_quotes flag was set to codes. The compact syntax `||` worked
fine in both modes.

Root cause: The spaced bar validation only accepted CompleteString and
PartialString terms, but in codes mode "abc" becomes Term::Cons([97,98,99]).

Fix: Add Term::Cons and empty list handling to spaced bar validation,
matching the compact || validation logic.

Also:
- Add discontiguous(test/2) directive to double_bar.pl (needed because
  set_prolog_flag directives appear between test clauses)
- Add double_bar_tests.stdout for proper CLI test registration

Author: jjtolton

src/parser/parser.rs

@@ -1094,10 +1094,14 @@ impl<'a, R: CharRead> Parser<'a, R> {
                         }
                     }
 
+                    // Check that the last term is a string literal (CompleteString, PartialString, or Cons from codes mode)
+                    // Must match the validation for compact || below
                     let is_valid = if let Some(last_term) = self.terms.last() {
                         match last_term {
                             Term::CompleteString(_, _) => true,
                             Term::PartialString(_, _, _) => true,
+                            Term::Cons(_, _, _) => true,  // Allows codes mode: "abc" becomes [97,98,99]
+                            Term::Literal(_, Literal::Atom(atom)) if *atom == atom!("[]") => true,  // Empty string in codes mode
                             _ => false,
                         }
                     } else {

src/tests/double_bar.pl

@@ -1,6 +1,7 @@
 :- module(double_bar_tests, []).
 
 :- use_module(test_framework).
+:- discontiguous(test/2).
 
 % Tests for the double bar || operator
 % Based on: https://www.complang.tuwien.ac.at/ulrich/iso-prolog/double_bar

tests/scryer/cli/src_tests/double_bar_tests.stdout

@@ -0,0 +1 @@
+All tests passed