Make error-status assertions authoritative

Replace standalone message assertions with a helper that checks the returned akerr status first, then validates message content only as secondary context. Drop ambiguous memcpy message checks where both NULL guards use the same format string.

tests/test_linkedlist.c

@@ -190,8 +190,9 @@ static int test_iterate_propagates_callback_error(void)
     visitlog_init(&log);
     log.fail_at = 0;
 
-    AKSL_CHECK_STATUS(aksl_list_iterate(&node, &record_visit, &log), AKERR_VALUE);
-    AKSL_CHECK_MSG_CONTAINS("iterator failed at visit 0");
+    AKSL_CHECK_STATUS_MSG_CONTAINS(
+        aksl_list_iterate(&node, &record_visit, &log),
+        AKERR_VALUE, "iterator failed at visit 0");
     AKSL_CHECK(log.count == 1);
     return 0;
 }