[pmap] Deprecate setting `jax_pmap_shmap_merge` config state.
Setting the `jax_pmap_shmap_merge` config state is deprecated in JAX v0.9.0
and will be removed in JAX v0.10.0. A deprecation warning will be emitted
whenever the config is set, regardless of the value.
NOTES
- Move `PmapShmapMergeTest` to its own target because we need decorators to avoid "global config state has changed" errors and these are called at class definition time.
- Introduce a `pmap_shmap_merge` flag in favor of the config-parsed `jax_pmap_shmap_merge` flag for pmap_test to explicitly manage how the config is updated.
PiperOrigin-RevId: 856657754
