Bug: Include PostProcessingAction in CountAsync

### Context:
`PostProcessingAction` is documented to facilitate in-memory tranformations or filtering:
https://specification.ardalis.com/features/postprocessingaction.html

### Issue:
In a simple scenario we have a `Fullname` computed property which is built from `Firstname` and `Lastname` columns. We use `PostProcessingAction` to implement filtering functionality on the `Fullname`.

Our `ListAsync` call handles this perfectly by filtering the result set correctly. However, our accompanying `CountAsync` ignores the `PostProcessingAction` and gives an incorrect totalcount for our paging implementation.

This does seem to be the intended implementation when looking at this: https://github.com/ardalis/Specification/issues/134#issuecomment-872528093

### Expected result:
`ListAsync` and `CountAsync` should resolve to identical result sets. Since `PostProcessingAction` is documented to facilitate in-memory filtering I expect the `CountAsync` to also resolve the `PostProcessingAction` to respect any in-memory filtering implementations to return the correct count.

Start by comparing the ListAsync and CountAsync paths for PostProcessingAction, then read the linked PostProcessingAction documentation and referenced issue #134. Done means CountAsync respects the same in-memory filtering behavior as ListAsync and returns a matching total for paging, with tests covering the computed Fullname scenario.

Written by the indexing model from the issue text.