Address review: use DYLD_LIBRARY_PATH on macOS, fix trailing slash

Gate the library path variable on APPLE so macOS picks up
DYLD_LIBRARY_PATH instead of the ineffective LD_LIBRARY_PATH, and
add the missing trailing slash on the gz_msgs_gen/python PYTHONPATH
entry to match the line above.

Generated-by: Claude <noreply@anthropic.com>
Signed-off-by: Taylor Howard <taylorhoward@me.com>

Author: taylorhoward92

python/CMakeLists.txt

@@ -26,9 +26,14 @@ if (BUILD_TESTING AND NOT WIN32)
     endif()
     set(_env_vars)
     list(APPEND _env_vars "PYTHONPATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/python/")
-    list(APPEND _env_vars "PYTHONPATH=path_list_prepend:${CMAKE_BINARY_DIR}/gz_msgs_gen/python")
-    list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
-    list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_BINARY_DIR}/lib")
+    list(APPEND _env_vars "PYTHONPATH=path_list_prepend:${CMAKE_BINARY_DIR}/gz_msgs_gen/python/")
+    if(APPLE)
+      list(APPEND _env_vars "DYLD_LIBRARY_PATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+      list(APPEND _env_vars "DYLD_LIBRARY_PATH=path_list_prepend:${CMAKE_BINARY_DIR}/lib")
+    else()
+      list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
+      list(APPEND _env_vars "LD_LIBRARY_PATH=path_list_prepend:${CMAKE_BINARY_DIR}/lib")
+    endif()
     set_tests_properties(${test}.py PROPERTIES
       ENVIRONMENT_MODIFICATION "${_env_vars}")
   endforeach()