gguf : add string length accessor
Add gguf_get_arr_str_len() to obtain the length of a string stored in a
GGUF string array without scanning for the terminator. This allows
callers to pre-compute the exact size of a result buffer before
formatting string arrays (ref:
https://github.com/ggml-org/llama.cpp/pull/30679#discussion_r4324797291).
Also bounds-check the index in gguf_get_arr_str().
Assisted-by: pi:llama.cpp/ggml-org/Qwen3.8-Flash-Next-GGUF (RTX 5090 32GB, DDR5 128GB)
