[CI] FIx `fixture 'siglip_attention_config' not found` (#31053)

Signed-off-by: Lucas Wilkinson <lwilkins@redhat.com>

tests/models/multimodal/pooling/conftest.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""Pytest configuration for vLLM pooling tests."""
+
+import pytest
+
+from vllm.platforms import current_platform
+
+
+@pytest.fixture
+def siglip_attention_config():
+    """Return attention config for SigLIP tests on ROCm.
+
+    On ROCm, SigLIP tests require FLEX_ATTENTION backend.
+    """
+    if current_platform.is_rocm():
+        return {"backend": "FLEX_ATTENTION"}
+    return None