llama-fit-params: keep explicit --ctx-size 0 (#19070)

include/llama.h

@@ -489,6 +489,7 @@ extern "C" {
     //   - returns true if the parameters could be successfully modified to fit device memory
     //   - this function is NOT thread safe because it modifies the global llama logger state
     //   - only parameters that have the same value as in llama_default_model_params are modified
+    //     with the exception of the context size which is modified if and only if equal to 0
     LLAMA_API enum llama_params_fit_status llama_params_fit(
                                    const char   * path_model,
                     struct llama_model_params   * mparams,