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/mtmd/mtmd.h

@@ -67,6 +67,7 @@ struct mtmd_batch;
 
 struct mtmd_input_text {
     const char * text;
+    size_t text_len;
     bool add_special;
     bool parse_special;
 };