COMPLETED

Task J: Add tests for the new --multi-value-return compiler flag

Summary:
- Created pkg/preprocessor/config_test.go with 10 comprehensive test functions
- Fixed build error in existing preprocessor_test.go (removed invalid SourceMap fields)
- All tests pass successfully (0.189s execution time)
- Test coverage: DefaultConfig(), ValidateMultiValueReturnMode(), full mode (2 tests), single mode (4 tests), nil config, assignments

Test Results: ✅ PASS (10/10 tests)
Files Created: 1 (config_test.go - 209 lines)
Files Modified: 1 (preprocessor_test.go - fixed build error)
