Relax jax dependency constraints to be able to install RC wheels
Also, add a job to the release test workflow that verifies that the release wheels can be installed.
TESTED:
1. Full release: https://github.com/jax-ml/jax/actions/runs/14315832784
2. jax only release: https://github.com/jax-ml/jax/actions/runs/14316157252
PiperOrigin-RevId: 744857804
