server: use random media marker (#21962)

* server: use random media marker

* nits

* remove legacy <__image__> token

* revert special char in random

tools/mtmd/mtmd.cpp

@@ -109,7 +109,7 @@ mtmd_context_params mtmd_context_params_default() {
         /* use_gpu           */ true,
         /* print_timings     */ true,
         /* n_threads         */ 4,
-        /* image_marker      */ MTMD_DEFAULT_IMAGE_MARKER,
+        /* image_marker      */ nullptr,
         /* media_marker      */ mtmd_default_marker(),
         /* flash_attn_type   */ LLAMA_FLASH_ATTN_TYPE_AUTO,
         /* warmup            */ true,
@@ -169,7 +169,7 @@ struct mtmd_context {
         media_marker (ctx_params.media_marker),
         n_embd_text  (llama_model_n_embd_inp(text_model))
     {
-        if (std::string(ctx_params.image_marker) != MTMD_DEFAULT_IMAGE_MARKER) {
+        if (ctx_params.image_marker != nullptr) {
             throw std::runtime_error("custom image_marker is not supported anymore, use media_marker instead");
         }
 
@@ -584,9 +584,6 @@ struct mtmd_tokenizer {
         parse_special = text->parse_special;
         input_text    = text->text;
         vocab         = llama_model_get_vocab(ctx->text_model);
-
-        // for compatibility, we convert image marker to media marker
-        string_replace_all(input_text, MTMD_DEFAULT_IMAGE_MARKER, ctx->media_marker);
     }
 
     int32_t tokenize(mtmd_input_chunks * output) {