Fix for bug in usePaginatedQuery_experimental (#54981)

Previously when a split was handled, it erroneously started the first split page at the start of the dataset (w/ a null cursor) instead of using the cursor for the page being split. If it was anything other than the first page being split, that would lead to duplicated items across the pages that the client was subscribed to.

That's fixed now and the fix also includes a test with a fake that stands in for a server that responds to split requests. That lets us test that the client gets proper results back for proper behavior rather than just asserting that it sends the correct arguments.

This addresses https://github.com/get-convex/convex-backend/issues/508.

GitOrigin-RevId: 7385cd72823f4aa9b56cf6c7fa0a42867c2892ae