The debug_storageRangeAt RPC method requires txIndex as a mandatory
parameter, meaning callers can only query state before a specific
transaction in a block. There is no way to query the post-block state
(after the last transaction), unlike other debug methods such as
debug_traceCall which allow omitting the index.
Make txIndex a pointer (*int) so it becomes optional:
- When provided: use stateAtTransaction to get state before that tx
(existing behavior, unchanged)
- When omitted (null): use stateAtBlock to get the post-block state
This is consistent with how other debug API endpoints handle optional
state references.
Fixes#31344