Configure OpenFHE for sparse packing properly

In 5bff766e838b5e9b49ed78339aafc8ab5a4b5113 we have the lattigo pipeline properly configure lattigo to use its sparse encoding (when the number of actual used slots `n` divides the max number of slots `N`). In Lattigo you do this by setting `LogDimensions` on a plaintext before encoding, and for bootstrappng this has to be paired with a corresponding setting of `LogSlots` on the bootstrapping params to use sparse bootstrapping.

OpenFHE has a similar situation, but in our current backend we aren't properly configuring the sparse packing, and are instead always assuming a full packing: https://github.com/google/heir/blob/26ef485a8ac9367ed4ff4e4f188d0d0129f1dd0c/lib/Target/OpenFhePke/OpenFhePkeEmitter.cpp#L1942

In OpenFHE the `SetBatchSize` is supposed to be used for this purpose, and now at the end of the pipeline we should have a [requested slot count attribute](https://github.com/google/heir/blob/26ef485a8ac9367ed4ff4e4f188d0d0129f1dd0c/lib/Dialect/ModuleAttributes.h#L25) on the module that corresponds to the actual number of slots needed.

This issue is to use that attribute to properly `SetBatchSize`, and to fix up any lingering assumptions in the OpenFHE emitter; e.g., [explicit cyclic repetition](https://github.com/google/heir/blob/26ef485a8ac9367ed4ff4e4f188d0d0129f1dd0c/lib/Target/OpenFhePke/OpenFhePkeEmitter.cpp#L1649) that should not be necessary if `SetBatchSize` is used properly, as well as various places where we query `GetRingDimension() / 2` to get a slot count. I had originally tried to do this in `5bff766e838b5e9b49ed78339aafc8ab5a4b5113`, and ran into errors that I didn't have time to fix.