Expose a negative_prompt string parameter on the image endpoints,
matching Stable Diffusion WebUI / vLLM-Omni conventions. When both the
negative_prompt parameter and a '|'-suffixed negative prompt in the
main prompt are present, they are joined with a comma so callers can
keep a global negative prompt in negative_prompt and add per-image
negative tags after '|'.
Assisted-by: Pi: DeepSeek V4 Pro
Signed-off-by: Fedor Zuev <Fedor.Zuev@gmail.com>