fix: populate participating_peers to enable correct peer penalization

[sashass1315]

Backfill intended to penalize only peers that actually participated in serving batches, but participating_peers was never populated. This meant no peers were penalized on terminal faulty failure. This change records the block peer on successful batch downloads in on_block_response() and records the peer on error in inject_error() after registering a failed download. This restores the intended behavior without altering APIs or broader flow.