science/rdkit: update 2025_09_2 → 2026.03.6
also:
- fixed many tests and upstreamed fixes (their testsuite is in a disarray, we shouldn't really be fixing people's tests, but anyways)
- discovered and fixed wrong clang detection in the code, reported upstream
- fixed wrongfully excluded code that caused plist issues
- fixed compiler crash on 16 (by using llvm:23)
(cherry picked from commit bba96c42a29fc0191b9513dde1e0ff90f8ddcc5b)