Fix test discovery failure: defer gtest_discover_tests to test time

gtest_discover_tests() runs the test binary at build time to enumerate
tests, but the CI container has no NVIDIA driver so libnvidia-ml.so.1
is missing at runtime. The binary links fine against the CUDA toolkit
stub but cannot execute.

- Use DISCOVERY_MODE PRE_TEST to defer discovery to ctest runtime
- Add LD_LIBRARY_PATH for CUDA stubs in the unit-tests CI job so the
  tests can load when ctest actually runs them

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

Author: DannyWall92

.github/workflows/ci-probe-agent.yml

@@ -78,6 +78,8 @@ jobs:
 
       - name: Build and test
         working-directory: probe-agent
+        env:
+          LD_LIBRARY_PATH: /usr/local/cuda/lib64/stubs
         run: |
           cmake -B build -G Ninja -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTING=ON
           cmake --build build --parallel $(nproc)

probe-agent/CMakeLists.txt

@@ -245,7 +245,7 @@ if(SENTINEL_BUILD_TESTS OR BUILD_TESTING)
             list(APPEND _UNIT_TEST_LIBS GTest::gmock)
         endif()
         target_link_libraries(sentinel_unit_tests PRIVATE ${_UNIT_TEST_LIBS})
-        gtest_discover_tests(sentinel_unit_tests)
+        gtest_discover_tests(sentinel_unit_tests DISCOVERY_MODE PRE_TEST)
 
         # CUDA unit tests
         set(CUDA_UNIT_TESTS
@@ -262,7 +262,7 @@ if(SENTINEL_BUILD_TESTS OR BUILD_TESTING)
                 GTest::gtest
                 GTest::gtest_main
         )
-        gtest_discover_tests(sentinel_cuda_unit_tests)
+        gtest_discover_tests(sentinel_cuda_unit_tests DISCOVERY_MODE PRE_TEST)
 
         # Integration tests (gRPC tests only when protobuf/gRPC are available)
         set(INTEGRATION_TESTS
@@ -281,7 +281,7 @@ if(SENTINEL_BUILD_TESTS OR BUILD_TESTING)
                 GTest::gtest
                 GTest::gtest_main
         )
-        gtest_discover_tests(sentinel_integration_tests)
+        gtest_discover_tests(sentinel_integration_tests DISCOVERY_MODE PRE_TEST)
     else()
         message(STATUS "GTest not found — skipping test targets")
     endif()