Disable FFTO by default in 3.1.x, keep enabled in 4.0.x (#5539)

Gate the ffto_enabled default on PROXYSQLGENAI so that:
- 3.1.x (PROXYSQL31): FFTO compiled in but disabled by default
- 4.0.x (PROXYSQLGENAI): FFTO enabled by default (unchanged)

Users on 3.1.x can still opt in via SET mysql-ffto_enabled='true'.

lib/MySQL_Thread.cpp

@@ -1362,7 +1362,11 @@ MySQL_Threads_Handler::MySQL_Threads_Handler() {
 	variables.ping_timeout_server=200;
 	variables.fast_forward_grace_close_ms=5000;
 #ifdef PROXYSQLFFTO
+#ifdef PROXYSQLGENAI
 	variables.ffto_enabled=true;
+#else
+	variables.ffto_enabled=false;
+#endif
 	variables.ffto_max_buffer_size=1048576;
 #endif
 	variables.default_schema=strdup((char *)"information_schema");

lib/PgSQL_Thread.cpp

@@ -1039,7 +1039,11 @@ PgSQL_Threads_Handler::PgSQL_Threads_Handler() {
 	variables.shun_on_failures = 5;
 	variables.shun_recovery_time_sec = 10;
 #ifdef PROXYSQLFFTO
+#ifdef PROXYSQLGENAI
 	variables.ffto_enabled = true;
+#else
+	variables.ffto_enabled = false;
+#endif
 	variables.ffto_max_buffer_size = 1048576;
 #endif
 	variables.unshun_algorithm = 0;