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 [email protected]