vocab : prevent tokenizer overflow (#14301)

* vocab : prevent stack overflow in tokenize

* vocab : return error instead of aborting on oversized token count

* vocab : INT32_MIN from llama_tokenize on overflow

include/llama.h

@@ -1088,6 +1088,7 @@ extern "C" {
     /// @param tokens The tokens pointer must be large enough to hold the resulting tokens.
     /// @return Returns the number of tokens on success, no more than n_tokens_max
     /// @return Returns a negative number on failure - the number of tokens that would have been returned
+    /// @return Returns INT32_MIN on overflow (e.g., tokenization result size exceeds int32_t limit)
     /// @param add_special Allow to add BOS and EOS tokens if model is configured to do so.
     /// @param parse_special Allow tokenizing special and/or control tokens which otherwise are not exposed and treated
     ///                      as plaintext. Does not insert a leading space.