fix(diffusers): keep negative_prompt unless negative embeds are set

The sd_embed branch dropped negative_prompt for every pipeline, which
discarded the user's negative prompt for Flux, where no negative embeds
are produced. Only drop it when negative_prompt_embeds is present, and
move the check to a helper that has unit tests.

Assisted-by: Claude Code:claude-sonnet-5-5
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>

backend/python/diffusers/backend.py

@@ -36,7 +36,7 @@ from diffusers_dynamic_loader import (
     get_available_pipelines,
     load_diffusers_pipeline,
 )
-from load_options import single_file_load_kwargs
+from load_options import drop_negative_prompt_if_embedded, single_file_load_kwargs
 
 # Import specific items still needed for special cases and safety checker
 from diffusers import DiffusionPipeline, ControlNetModel
@@ -882,9 +882,9 @@ class BackendServicer(backend_pb2_grpc.BackendServicer):
                     prompt = prompt,
                 )
 
-            # remove negative_prompt that may have been added earlier from options to avoid conflict
-            # errors from passing both negative_prompt and negative_prompt_embeds
-            kwargs.pop('negative_prompt', None)
+            # negative_prompt conflicts with negative_prompt_embeds; pipelines that
+            # got no negative embeds (e.g. Flux) keep the user's negative prompt
+            drop_negative_prompt_if_embedded(kwargs)
 
             image = self.pipe(
                 guidance_scale=self.cfg_scale,