feat: set flow_shift as sample param to match upstream stable-diffusion.cpp

According to owner feedback, flow_shift became a sample param (sd_sample_params_t)
instead of a context param (sd_ctx_params_t). Added the flow_shift assignment
to sample_params to match the upstream library at commit d950627.

Author: localai-bot

backend/go/stablediffusion-ggml/gosd.cpp

@@ -872,6 +872,7 @@ int gen_image(sd_img_gen_params_t *p, int steps, char *dst, float cfg_scale, cha
     p->sample_params.sample_method = sample_method;
     p->sample_params.sample_steps = steps;
     p->sample_params.scheduler = scheduler;
+    p->sample_params.flow_shift = INFINITY;
 
     int width = p->width;
     int height = p->height;