[MLIR][Python] use nb::typed for return signatures (#160221)
https://github.com/llvm/llvm-project/pull/160183 removed `nb::typed`
annotation to fix bazel but it turned out to be simply a matter of not
using the correct version of nanobind (see
https://github.com/llvm/llvm-project/pull/160183#issuecomment-3321429155).
This PR restores those annotations but (mostly) moves to the return
positions of the actual methods.