tests : refactor test-save-load-state to accept token input (#24073)

tests : refactor test-save-load-state to accept token input (#24073)

* tests : refactor test-save-load-state to accept token input

- Default prompt is now empty; when not provided, generate n_batch
  random tokens (useful for models without a tokenizer)
- Tokenization happens once upfront; pass token vector to test functions
- generate_tokens prints token IDs instead of decoded pieces
- Use llama_model_get_vocab / llama_vocab_n_tokens API
- Upgrade log level from LOG_TRC to LOG_INF for visibility

Assisted-by: llama.cpp:local pi
* cont : use llama_tokens alias