## Issue Addressed
In #4027 I forgot to add the `parent_block_number` to the payload attributes SSE.
## Proposed Changes
Compute the parent block number while computing the pre-payload attributes. Pass it on to the SSE stream.
## Additional Info
Not essential for v3.5.1 as I suspect most builders don't need the `parent_block_root`. I would like to use it for my dummy no-op builder however.
36e163c042