test: replace dall-e-3 with gpt-image-1 in health check and router tests (#27813)

OpenAI returns 'The model dall-e-3 does not exist' for the test account,
breaking test_openai_img_gen_health_check and test_image_generation.
Switch to gpt-image-1, matching the existing TestOpenAIGPTImage1 pattern.

tests/litellm_utils_tests/test_health_check.py

@@ -68,7 +68,7 @@ async def test_azure_embedding_health_check():
 async def test_openai_img_gen_health_check():
     response = await litellm.ahealth_check(
         model_params={
-            "model": "dall-e-3",
+            "model": "gpt-image-1",
             "api_key": os.getenv("OPENAI_API_KEY"),
         },
         mode="image_generation",

tests/router_unit_tests/test_router_helper_utils.py

@@ -40,9 +40,9 @@ def model_list():
             },
         },
         {
-            "model_name": "dall-e-3",
+            "model_name": "gpt-image-1",
             "litellm_params": {
-                "model": "dall-e-3",
+                "model": "gpt-image-1",
                 "api_key": os.getenv("OPENAI_API_KEY"),
             },
         },
@@ -207,12 +207,12 @@ async def test_image_generation(model_list, sync_mode):
     router = Router(model_list=model_list)
     if sync_mode:
         response = router._image_generation(
-            model="dall-e-3",
+            model="gpt-image-1",
             prompt="A cute baby sea otter",
         )
     else:
         response = await router._aimage_generation(
-            model="dall-e-3",
+            model="gpt-image-1",
             prompt="A cute baby sea otter",
         )
 
@@ -382,7 +382,7 @@ async def test_router_make_call(model_list):
     ## AIMAGE_GENERATION
     response = await router.make_call(
         original_function=router._aimage_generation,
-        model="dall-e-3",
+        model="gpt-image-1",
         prompt="A cute baby sea otter",
         mock_response="https://example.com/image.png",
     )