fix(store): reject oversized nullifier sync pages

[UfukNode]

Summary

select_nullifiers_by_prefix used to drop the truncation block when a response exceeded MAX_ROWS. If every returned row came from that same block, it returned an empty page with a cursor before the block, so clients could keep asking for the same range forever.

This now returns an explicit payload-limit error for that case, matching the transaction sync behavior. Added a regression test that inserts MAX_ROWS + 1 matching nullifiers in one block.

Closes #2460.

Changelog

[[entry]]
scope       = "rpc"
impact      = "fixed"
description = "Return an explicit error instead of stalling nullifier sync when one block exceeds the response payload limit."