Add comprehensive tests for g_caused_exception/2

Following TESTING_GUIDE.md, added tests at layers 2 and 3:

Layer 2 - Prolog Integration Tests (src/tests/custom_toplevel.pl):
- Test that g_caused_exception/2 is not asserted when no exception occurs
- Test that g_caused_exception/2 can be checked from custom toplevel
- Added check_for_exception/0 helper predicate for testing

Layer 3 - CLI Tests (tests/scryer/cli/src_tests/custom_toplevel.md):
- Test g_caused_exception/2 with exception thrown
- Test g_caused_exception/2 with no exception
- Test g_caused_exception/2 with error/2 terms
- Added test helper predicates in fixtures/toplevel_test_helper.pl

All tests pass successfully.

Co-Authored-By: J.J.'s Robot <jjtolton@gmail.com>

tests/scryer/cli/fixtures/toplevel_test_helper.pl

@@ -0,0 +1,50 @@
+% Helper predicates for testing custom toplevel functionality
+
+success_toplevel :-
+    write('SUCCESS_TOPLEVEL_EXECUTED'), nl,
+    halt(0).
+
+failure_toplevel :-
+    write('FAILURE_TOPLEVEL_EXECUTED'), nl,
+    halt(1).
+
+exit_code_42 :-
+    write('EXIT_CODE_42'), nl,
+    halt(42).
+
+write_and_exit :-
+    write('Output from custom toplevel'), nl,
+    halt(0).
+
+% This one doesn't halt - to test what happens if toplevel doesn't halt
+non_halting_toplevel :-
+    write('NON_HALTING_TOPLEVEL'), nl.
+
+% Test that toplevel can access loaded predicates
+test_file_loaded :-
+    write('LOADED_PREDICATE_CALLED'), nl,
+    halt(0).
+
+helper_predicate :-
+    write('Helper predicate works'), nl.
+
+% g_caused_exception/2 testing predicates
+:- dynamic(g_caused_exception/2).
+
+check_exception_halt_1 :-
+    (   g_caused_exception(Goal, Exception) ->
+        write('EXCEPTION_CAUGHT'), nl,
+        write('Goal: '), write(Goal), nl,
+        write('Exception: '), write(Exception), nl,
+        halt(1)
+    ;   write('NO_EXCEPTION'), nl,
+        halt(0)
+    ).
+
+check_exception_halt_0 :-
+    (   g_caused_exception(_, _) ->
+        write('UNEXPECTED_EXCEPTION'), nl,
+        halt(1)
+    ;   write('SUCCESS_NO_EXCEPTION'), nl,
+        halt(0)
+    ).