[Attention] Update tests to remove deprecated env vars (#30563)

Signed-off-by: Matthew Bonanni <mbonanni@redhat.com>

tests/models/multimodal/pooling/conftest.py

@@ -2,23 +2,17 @@
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 """Pytest configuration for vLLM pooling tests."""
 
-import os
-import warnings
+import pytest
 
 from vllm.platforms import current_platform
 
 
-def pytest_collection_modifyitems(config, items):
-    """Set FLEX_ATTENTION backend for SigLIP tests on ROCm."""
-    if not current_platform.is_rocm():
-        return
+@pytest.fixture
+def siglip_attention_config():
+    """Return attention config for SigLIP tests on ROCm.
 
-    siglip_tests = [item for item in items if "test_siglip" in item.nodeid]
-
-    if siglip_tests:
-        os.environ["VLLM_ATTENTION_BACKEND"] = "FLEX_ATTENTION"
-        warnings.warn(
-            "ROCm: Set VLLM_ATTENTION_BACKEND=FLEX_ATTENTION for SigLIP tests",
-            UserWarning,
-            stacklevel=1,
-        )
+    On ROCm, SigLIP tests require FLEX_ATTENTION backend.
+    """
+    if current_platform.is_rocm():
+        return {"backend": "FLEX_ATTENTION"}
+    return None