testing_commitBlockV1 is the write companion of testing_buildBlockV1: it
builds a block from the provided payloadAttributes and txs, inserts it,
and sets it as the canonical head, returning the new head hash. Skipping
the engine_newPayload + engine_forkchoiceUpdated serialize/deserialize
round-trip makes it useful for state-shape benchmarking and reproducible
test-chain construction where the caller wants the chain to advance.
The new miner.CommitTestingBlock shares its generation path with
BuildTestingPayload via an unexported helper, so both code paths produce
the same block from the same inputs.
Spec and cross-client fixtures: ethereum/execution-apis#801
This is a copy of #34721 but against `master` (rather than
`bal-devnet-3`), as requested by @jwasinger, since the slotnum logic now
exists on `master` as well.
