Wait for LightClientFinalityUpdate exceeding LightClientUpdate

[yoshidan]

Summary

Fix data consistency issues between preimage-maker and relayer when fetching light client data from Beacon API.

Problem

When fetching light_client/finality_update and light_client/updates at different times, the following
inconsistencies could occur:

1. Period boundary mismatch: signature_slot and finalized_header.beacon.slot can belong to different periods
2. Passing slot: Finality can progress between the two API calls, causing updates.finalized_slot > finality_update.finalized_slot

Solution

• Add retry logic for the following conditions:
  • updates.finalized_slot > finality.finalized_slot (race condition)
  • signature_period != finalized_period (period boundary)