server: Expose media_tag on /props endpoint. (#22028)

tools/server/README.md

@@ -806,6 +806,7 @@ By default, it is read-only. To make POST request to change global properties, y
   "modalities": {
     "vision": false
   },
+  "media_marker": "<__media_YoNhud46VdDqbuFmKYEO9PY7A4ARzRfg__>",
   "build_info": "b(build number)-(build commit hash)",
   "is_sleeping": false
 }

tools/server/server-context.cpp

@@ -3537,6 +3537,7 @@ void server_routes::init_routes() {
                 {"vision", meta->has_inp_image},
                 {"audio",  meta->has_inp_audio},
             } },
+            { "media_marker",                get_media_marker() },
             { "endpoint_slots",              params.endpoint_slots },
             { "endpoint_props",              params.endpoint_props },
             { "endpoint_metrics",            params.endpoint_metrics },