feat: memoize for FFI_PartitionEvaluator

[sandugood]

Which issue does this PR close?

• Closes FFI: FFI_PartitionEvaluator missing memoize #22336 .

Rationale for this change

Before this change there was no reasonable memoize for the ForeignPartitionEvaluator

What changes are included in this PR?

1. Added memoize function pointer to FFI_PartitionEvaluator struct and implemented the FFI wrapper memoize_fn_wrapper that serializes/deserializes WindowAggState across the boundary

2. Implemented PartitionEvaluator::memoize for ForeignPartitionEvaluator, preserving window_frame_ctx (which cannot cross FFI) across the call

Are these changes tested?

Yes, tested both state preservation with test_memoize_state_preservation and window_frame_ctx preservation with test_memoize_preserves_window_frame_ctx. All of the other tests in the crate also pass

Are there any user-facing changes?

Yes, but only behavioral — no API changes.