[Pallas TPU] Add lowerings for bf16 `jnp.ceil` and `jnp.floor` in TPU v6+
This PR is similar to https://github.com/jax-ml/jax/pull/24284
Note that `np.testing.assert_allclose()` is changed to `self.assertAllClose()` because the latter is a wrapper with bfloat16 support.
PiperOrigin-RevId: 688581914