[Bugfix] Fix negative_prompt usage in z_image_examples.py

Move negative_prompt from OmniDiffusionSamplingParams to prompt dict.
OmniDiffusionSamplingParams does not have a negative_prompt field, so
passing it there causes TypeError at runtime. The correct pattern is to
pass negative_prompt in the prompt dict, matching text_to_image.py usage.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: dougbtv

examples/offline_inference/text_to_image/z_image_examples.py

@@ -32,13 +32,15 @@ def z_image_base_example():
     omni_base = Omni(model="Tongyi-MAI/Z-Image")
 
     outputs_base = omni_base.generate(
-        "a majestic mountain landscape at sunset, detailed, photorealistic",
+        {
+            "prompt": "a majestic mountain landscape at sunset, detailed, photorealistic",
+            "negative_prompt": "blurry, low quality, distorted, oversaturated",
+        },
         OmniDiffusionSamplingParams(
             height=1280,
             width=720,
             num_inference_steps=50,
             guidance_scale=4.0,
-            negative_prompt="blurry, low quality, distorted, oversaturated",
             seed=42,
         ),
     )
@@ -93,9 +95,9 @@ def batch_inference_example():
     omni = Omni(model="Tongyi-MAI/Z-Image")
 
     prompts = [
-        "a cup of coffee on a wooden table",
-        "a cat sleeping on a cozy blanket",
-        "a futuristic city skyline at night",
+        {"prompt": "a cup of coffee on a wooden table", "negative_prompt": "blurry, low quality"},
+        {"prompt": "a cat sleeping on a cozy blanket", "negative_prompt": "blurry, low quality"},
+        {"prompt": "a futuristic city skyline at night", "negative_prompt": "blurry, low quality"},
     ]
 
     # Note: These will be processed sequentially unless max_batch_size > 1
@@ -106,7 +108,6 @@ def batch_inference_example():
             width=1024,
             num_inference_steps=40,
             guidance_scale=4.0,
-            negative_prompt="blurry, low quality",
             seed=42,
         ),
     )