fix(trim): prevent assertion failure in trim function

[LaffeyNyaa]

When compiling with MSVC in the Debug configuration, during the execution of the llama_chat_apply_template function, the trim function (located in src/llama-chat.cpp) is called for certain models (e.g., Gemma 3). If non-ASCII characters are present in the const struct llama_chat_message * chat passed to the llama_chat_apply_template function, the assertion within the isspace function in the trim function will trigger an Assertion Failed error. The error message displays the expression: c >= -1 && c <= 255.

To resolve this problem, I added static_cast<unsigned char> to the isspace call, ensuring that negative-valued non-ASCII characters are converted to positive values.