mtmd: fix silent prompt truncation on embedded NUL (#25548)

* mtmd: fix silent prompt truncation on embedded NUL

mtmd_input_text carried the prompt as a bare const char* with no
length, so a NUL byte in message content cut the prompt at the
tokenizer boundary and dropped every later message plus the assistant
marker, with no log. Add an explicit text_len and thread it through,
matching llama_tokenize and the text only path.

* cleanup

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

tools/server/server-common.cpp

@@ -705,7 +705,8 @@ server_tokens process_mtmd_prompt(mtmd_context * mctx, const std::string & promp
     std::vector<server_tokens> inputs;
     // multimodal
     mtmd_input_text inp_txt = {
-        prompt.c_str(),
+        prompt.data(),
+        prompt.size(),
         /* add_special */   true,
         /* parse_special */ true,
     };