Build: forward-slash Python3_EXECUTABLE when generating rdkit-stubs (#9318)

The rdkit-stubs CMakeLists builds a cmake script (RUN_GEN_RDKIT_STUBS_PY)
that is later run via cmake -P. Every path embedded in that generated
script is converted to forward slashes first (CMAKE_SOURCE_DIR,
CMAKE_CURRENT_BINARY_DIR, PYTHON_INSTDIR) so the backslashes in native
Windows paths don't get treated as escape sequences when the script is
re-parsed.

Python3_EXECUTABLE was the one path that wasn't converted, so on Windows
the generated COMMAND line contained backslashes and failed to parse.
Apply the same forward-slash conversion to it for consistency.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

rdkit-stubs/CMakeLists.txt

@@ -14,13 +14,14 @@ set(CONCURRENCY "")
 if (DEFINED ENV{CMAKE_BUILD_PARALLEL_LEVEL})
   set(CONCURRENCY "--concurrency $ENV{CMAKE_BUILD_PARALLEL_LEVEL} ")
 endif()
+string(REGEX REPLACE "\\\\" "/" Python3_EXECUTABLE_FWDSLASH ${Python3_EXECUTABLE})
 string(CONCAT RUN_GEN_RDKIT_STUBS_PY "message("
   "\"-- Building and Installing rdkit-stubs into the following director${DIR_OR_DIRS}: ${STUB_LOCATIONS}\")\n"
   "set (SEPARATOR \"=====================================================================\n\")\n"
   "set (COMMON_FILENAME ${CMAKE_CURRENT_BINARY_DIR_FWDSLASH}/gen_rdkit_stubs)\n"
   "execute_process("
   "COMMAND "
-  "${Python3_EXECUTABLE} -m Scripts.gen_rdkit_stubs ${CONCURRENCY}"
+  "${Python3_EXECUTABLE_FWDSLASH} -m Scripts.gen_rdkit_stubs ${CONCURRENCY}"
   "${CMAKE_SOURCE_DIR_FWDSLASH_QUOTED}${PYTHON_INSTDIR_FWDSLASH_QUOTED}"
   "WORKING_DIRECTORY \"${CMAKE_SOURCE_DIR_FWDSLASH}\" "
   "OUTPUT_FILE \"\${COMMON_FILENAME}.out\" "