Add comprehensive tests for g_caused_exception/2

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 <[email protected]>