fix: add missing string.h include in test_double_process.c

Co-authored-by: aider (deepseek/deepseek-coder) <aider@aider.chat>

test_double_process.c

@@ -3,6 +3,7 @@
 #include "engine.h"
 #include <stdio.h>
 #include <assert.h>
+#include <string.h>
 
 void test_command_processed_only_once(void) {
     printf("Test: Command processed only once...\n");