op-acceptance-tests: ELP2P for EL Syncing for unsafe gap

[pcw109550]

Description

The test TestL2ELP2PCanonicalChainAdvancedByFCU demonstrates the underlying L2EL(geth) behavior.

Insights:

• NewPayload alone never initiates ELP2P/EL sync, but can build a non canonical chain if state exists.
• FCU is the mechanism that (a) promotes non canonical chain blocks to canonical when they are already fully validated, and (b) triggers EL sync when ancestors are missing.
• Previously submitted NewPayloads that returned SYNCING are not retained to automatically assemble a non canonical chain later.
• With ELP2P enabled, repeated FCU attempts eventually validate and advance the canonical chain.

Therefore we may solely rely on multi-FCU calls to trigger and validate EL Sync.

Metadata

Part of #17694