Fix: Ensure deterministic sub-protocol registration in ConsensusSchedule

[saeeddawod]

This PR fixes a critical bug that prevents successful IBFT2 →
QBFT consensus migration in Besu. During migration, nodes
would stall and fail to produce blocks because they could only
speak one BFT wire protocol instead of both.

Problem

When running a consensus migration from IBFT2 to QBFT using a
transitions configuration in genesis, the network would stall
before reaching the fork block. Nodes could not exchange IBFT2
messages even though they were still running IBFT2 consensus
pre-fork.

Root Cause

In ConsensusScheduleBesuControllerBuilder.createSubProtocolCon figuration(), the original code was:

return besuControllerBuilderSchedule
    .get(besuControllerBuilderSchedule.keySet().stream().skip(1
).findFirst().orElseThrow())
    .createSubProtocolConfiguration(ethProtocolManager,
maybeSnapProtocolManager);

Issues with this approach:

1. Non-deterministic ordering:
   besuControllerBuilderSchedule is a Map (HashMap), so
   keySet().stream().skip(1) produces unpredictable results
   depending on hash ordering
2. Only one protocol registered: Only the sub-protocol
   configuration from ONE consensus mechanism was registered,
   meaning nodes could only speak QBFT on the wire while still
   needing IBFT2 consensus pre-fork
3. Peer communication failure: Nodes couldn't exchange
   IBFT2 messages → block production stalls before fork height

Solution

The fix registers all BFT sub-protocols from all scheduled
consensus mechanisms:

• IBF/1 (IBFT2) - for pre-migration communication
• istanbul/100 (QBFT) - for post-migration communication

This allows nodes to communicate using both protocols during
the transition period. The fix:

1. Iterates through all consensus builders (sorted by block
   number for determinism)
2. Merges their sub-protocol configurations
3. Avoids duplicates (e.g., EthProtocol which is common to
   both)

Testing

Migration Test Performed

1. Started IBFT2 network - 4 validator nodes producing
   blocks under IBFT2 consensus
2. Verified IBFT2 active - Logs showed
   IbftBesuControllerBuilder for blocks #0-49
3. Stopped network at block ~46 (before fork block 50)
4. Restarted with migration genesis containing both IBFT2
   and QBFT config with startBlock: 50
5. Verified seamless transition:
   • Blocks Fix default logging #47-49: IbftBesuControllerBuilder (IBFT2)
   • Block Fix some mark sweep pruner bugs where nodes that should be kept were being swept (Re-merge of #38) #50+: QbftBesuControllerBuilder (QBFT)
6. Confirmed continuous operation - Network continued
   producing blocks past [PIE-2026] Separate in-sync from sync-status listeners #100 under QBFT

Log Evidence

# Pre-fork (IBFT2)
IbftBesuControllerBuilder | Produced #49 / 0 tx / ...

# Fork block (QBFT takes over)
QbftBesuControllerBuilder | Imported empty block #50 / 0 tx /
...

# Post-fork (QBFT continues)
QbftBesuControllerBuilder | Produced empty block #73 / 0 tx /
...

Genesis Configuration Used

{
  "config": {
    "ibft2": {
      "blockperiodseconds": 1,
      "epochlength": 30000,
      "requesttimeoutseconds": 2
    },
    "qbft": {
      "blockperiodseconds": 1,
      "epochlength": 30000,
      "requesttimeoutseconds": 2,
      "startBlock": 50
    }
  }
}

Fixed Issue(s)

Fixes consensus migration failures when transitioning from
IBFT2 to QBFT using scheduled transitions.

Checklist

• Checked out [contribution guidelines](https://github.com/
  hyperledger/besu/blob/main/CONTRIBUTING.md)
• Considered documentation - doc-change-required may be
  needed to clarify migration requirements
• Considered changelog - This is a bug fix for consensus
  migration
• No database changes

Local Tests

• Manual migration test (IBFT2 → QBFT) - PASSED
• spotless: ./gradlew spotlessApply
• unit tests: ./gradlew build
• acceptance tests: ./gradlew acceptanceTest
• integration tests: ./gradlew integrationTest