fix(core): handle driver-version failure in test helpers

Test helpers calling driver_version() at module scope would crash
in no-driver environments before test collection. Mirror the
production lazy-probe pattern: catch exceptions and pass None.

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

Author: cpcloud

cuda_core/tests/test_linker.py

@@ -22,8 +22,12 @@
 
 def _current_env_backend() -> str:
     """Return the backend a default (PTX input, no LTO) Linker picks on this machine."""
+    try:
+        drv_major = driver_version()[0]
+    except Exception:
+        drv_major = None
     return _linker._choose_backend(
-        driver_version()[0],
+        drv_major,
         _linker._probe_nvjitlink(),
         inputs_have_ltoir=False,
         lto_requested=False,

cuda_core/tests/test_program.py

@@ -20,8 +20,12 @@
 
 def _default_linker_backend() -> str:
     """Backend a default (PTX input, no LTO) Linker picks on this machine."""
+    try:
+        drv_major = driver_version()[0]
+    except Exception:
+        drv_major = None
     return _linker._choose_backend(
-        driver_version()[0],
+        drv_major,
         _linker._probe_nvjitlink(),
         inputs_have_ltoir=False,
         lto_requested=False,