test(core): add GPU-free tests for Linker.backend classmethod

Tests monkeypatch module-level flags in cuda.core._linker to verify
the classmethod without requiring a GPU. Covers nvJitLink path,
driver path, probe invocation when not memoised, classmethod
descriptor check, and idempotency.

Refs #714

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: cpcloud

cuda_core/tests/test_linker_backend.py

@@ -0,0 +1,49 @@
+# SPDX-FileCopyrightText: Copyright (c) 2024-2026 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# SPDX-License-Identifier: LicenseRef-NVIDIA-SOFTWARE-LICENSE
+
+"""GPU-free tests for Linker.backend classmethod.
+
+These live in a separate file from test_linker.py because that module calls
+Device() at import time, which requires a GPU. These tests use monkeypatch
+to set module-level flags and never touch CUDA devices.
+"""
+
+import inspect
+
+import pytest
+
+import cuda.core._linker as _linker
+from cuda.core._linker import Linker
+
+
+class TestBackendClassmethod:
+    def test_backend_returns_nvjitlink(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", True)
+        assert Linker.backend() == "nvJitLink"
+
+    def test_backend_returns_driver(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", False)
+        assert Linker.backend() == "driver"
+
+    def test_backend_invokes_probe_when_not_memoised(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", None)
+        called = []
+
+        def fake_decide():
+            called.append(True)
+            return False  # False = not falling back to driver = nvJitLink
+
+        monkeypatch.setattr(_linker, "_decide_nvjitlink_or_driver", fake_decide)
+        result = Linker.backend()
+        assert result == "nvJitLink"
+        assert called, "_decide_nvjitlink_or_driver was not called"
+
+    def test_backend_is_classmethod(self):
+        attr = inspect.getattr_static(Linker, "backend")
+        assert isinstance(attr, classmethod)
+
+    def test_backend_idempotent(self, monkeypatch):
+        monkeypatch.setattr(_linker, "_use_nvjitlink_backend", True)
+        results = [Linker.backend() for _ in range(3)]
+        assert results == ["nvJitLink", "nvJitLink", "nvJitLink"]