[CI][Test] Mock CPU backend block sizes in kv_connector unit conftest (#56671)

Signed-off-by: Xiaozhen Bi <xiaozhen.bi@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

tests/v1/kv_connector/unit/conftest.py

@@ -0,0 +1,27 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import pytest
+
+from vllm.platforms import current_platform
+
+
+@pytest.fixture(autouse=True)
+def _mock_cpu_backend_block_sizes(monkeypatch):
+    """Mock the CPU attention backend's kernel block-size declaration.
+
+    Connector unit tests exercise transfer logic, never the attention
+    kernel, and hard-code block_size=16. On CPU-only machines
+    `CPUAttentionBackend` declares `MultipleOf(32)` (#54042), so every
+    worker construction would fail in `select_common_block_size` before
+    reaching the code under test.
+    """
+    if not current_platform.is_cpu():
+        return
+    from vllm.v1.attention.backend import MultipleOf
+    from vllm.v1.attention.backends.cpu_attn import CPUAttentionBackend
+
+    monkeypatch.setattr(
+        CPUAttentionBackend,
+        "get_supported_kernel_block_sizes",
+        staticmethod(lambda: [MultipleOf(16)]),
+    )