[ci] Switch some CI jobs to H200 MIG slices (#38956)

.buildkite/test_areas/entrypoints.yaml

@@ -61,6 +61,7 @@ steps:
 
 - label: Entrypoints Integration (API Server openai - Part 3)
   timeout_in_minutes: 50
+  device: h200_18gb
   working_dir: "/vllm-workspace/tests"
   source_file_dependencies:
   - vllm/
@@ -105,6 +106,7 @@ steps:
 
 - label: OpenAI API Correctness
   timeout_in_minutes: 30
+  device: h200_18gb
   source_file_dependencies:
   - csrc/
   - vllm/entrypoints/openai/