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/test_load_options.py

@@ -2,7 +2,10 @@
 
 import unittest
 
-from backend.python.diffusers.load_options import single_file_load_kwargs
+from backend.python.diffusers.load_options import (
+    drop_negative_prompt_if_embedded,
+    single_file_load_kwargs,
+)
 
 
 class SingleFileLoadKwargsTest(unittest.TestCase):
@@ -22,5 +25,24 @@ class SingleFileLoadKwargsTest(unittest.TestCase):
         self.assertEqual(single_file_load_kwargs("", True), {})
 
 
+class DropNegativePromptTest(unittest.TestCase):
+    def test_drops_negative_prompt_when_embeds_present(self):
+        kwargs = {"negative_prompt": "blurry", "negative_prompt_embeds": object()}
+        drop_negative_prompt_if_embedded(kwargs)
+        self.assertNotIn("negative_prompt", kwargs)
+        self.assertIn("negative_prompt_embeds", kwargs)
+
+    def test_keeps_negative_prompt_without_embeds(self):
+        kwargs = {"negative_prompt": "blurry", "prompt_embeds": object()}
+        drop_negative_prompt_if_embedded(kwargs)
+        self.assertEqual(kwargs["negative_prompt"], "blurry")
+
+    def test_no_negative_prompt_is_fine(self):
+        self.assertEqual(
+            drop_negative_prompt_if_embedded({"negative_prompt_embeds": 1}),
+            {"negative_prompt_embeds": 1},
+        )
+
+
 if __name__ == "__main__":
     unittest.main()