server: use random media marker (#21962)

* server: use random media marker

* nits

* remove legacy <__image__> token

* revert special char in random

tools/server/server-common.h

@@ -92,6 +92,9 @@ std::string random_string();
 std::string gen_chatcmplid();
 std::string gen_tool_call_id();
 
+// get a random marker; note: each time the server restarts, the marker will be different
+const char * get_media_marker();
+
 //
 // lora utils
 //