test: stub the CTK lookup in the build-dir capture helper

The two TestGeneratedSourceDirIsKeyed cases call the real
_build_cuda_core(), whose first statement resolves CUDA_PATH/CUDA_HOME
for the compiler's include dirs. The helper stubbed cythonize() but not
that lookup, so both tests failed on every (wheels) CI job -- those
install a prebuilt wheel and deliberately have no toolkit present:

    RuntimeError: Environment variable CUDA_PATH or CUDA_HOME is not set

Nothing in these tests needs a real toolkit. cythonize() is replaced so
no C++ is generated or compiled, the path is only string-joined into
include_dirs and never dereferenced, and the CUDA major comes from the
CUDA_CORE_BUILD_MAJOR env var the helper already sets, which
_determine_cuda_major_version() returns from before reading cuda.h.

Stub _get_cuda_path rather than dropping the call: it must stay first so
the cuda.pathfinder import repairs PEP 517 namespace shadowing before
cuda.bindings is imported, which
test_cuda_path_is_resolved_before_importing_bindings pins. That test is
also the precedent here -- it calls _build_cuda_core() and stubs the same
function, which is why it has been green in the wheels jobs all along.

Verified by reproducing the wheels condition with CUDA_PATH and CUDA_HOME
unset: 2 failed before, 23 passed after, and 23 passed with them set.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: rparolin

cuda_core/tests/test_build_hooks.py

@@ -226,6 +226,9 @@ def fake_cythonize(ext_modules, **kwargs):
         captured.update(kwargs)
         return []
 
+    # Builds resolve the CTK for include dirs; stub it so the test runs
+    # where no toolkit is installed (e.g. the wheels CI jobs).
+    monkeypatch.setattr(build_hooks, "_get_cuda_path", lambda: "/nonexistent-cuda")
     monkeypatch.setattr(build_hooks, "cythonize", fake_cythonize)
     monkeypatch.setenv("CUDA_CORE_BUILD_MAJOR", cuda_major)
     build_hooks._determine_cuda_major_version.cache_clear()