feat: Add image_size field for Gemini Developer API Imagen 4 generation

PiperOrigin-RevId: 789937547

Author: matthew29tang

google/genai/models.py

@@ -997,7 +997,11 @@ def _GenerateImagesConfig_to_mldev(
     raise ValueError('add_watermark parameter is not supported in Gemini API.')
 
   if getv(from_object, ['image_size']) is not None:
-    raise ValueError('image_size parameter is not supported in Gemini API.')
+    setv(
+        parent_object,
+        ['parameters', 'sampleImageSize'],
+        getv(from_object, ['image_size']),
+    )
 
   if getv(from_object, ['enhance_prompt']) is not None:
     raise ValueError('enhance_prompt parameter is not supported in Gemini API.')

google/genai/tests/models/test_generate_images.py

@@ -44,7 +44,6 @@
     ),
     pytest_helper.TestTableItem(
         name='test_generate_size_2k_imagen4-001',
-        exception_if_mldev='not supported in Gemini API',
         parameters=types._GenerateImagesParameters(
             model='imagen-4.0-generate-001',
             prompt='Red skateboard',