fix(fast-inbox): register Inbox buckets in the prefilled mock archiver (A-1384)

The world-state integration test drives the streaming synchronizer through MockPrefilledArchiver, but
setPrefilled only seeded the legacy per-checkpoint message map and never registered Inbox buckets. Post-flip
the synchronizer reconstructs each block's consumed L1-to-L2 bundle from buckets via
getInboxBucketByTotalMsgCount, which returned undefined against the empty bucket map, so every block synced
with an empty bundle and its reconstructed state diverged from the committed header (block state does not
match world state). Register a genesis sentinel bucket plus one bucket per message-carrying checkpoint with
cumulative totalMsgCount matching the block's post-insertion leaf count.

Author: spalladino

yarn-project/archiver/src/test/mock_archiver.ts

@@ -1,5 +1,5 @@
 import type { CheckpointNumber } from '@aztec/foundation/branded-types';
-import type { Fr } from '@aztec/foundation/curves/bn254';
+import { Fr } from '@aztec/foundation/curves/bn254';
 import type { L2BlockSource } from '@aztec/stdlib/block';
 import type { Checkpoint } from '@aztec/stdlib/checkpoint';
 import type { InboxBucket, L1ToL2MessageSource } from '@aztec/stdlib/messaging';
@@ -51,6 +51,7 @@ export class MockArchiver extends MockL2BlockSource implements L2BlockSource, L1
  */
 export class MockPrefilledArchiver extends MockArchiver {
   private prefilled: Checkpoint[] = [];
+  private prefilledMessages: Fr[][] = [];
 
   constructor(prefilled: { checkpoint: Checkpoint; messages: Fr[] }[]) {
     super();
@@ -65,6 +66,51 @@ export class MockPrefilledArchiver extends MockArchiver {
       }
       this.setL1ToL2Messages(checkpoint.number, messages);
     }
+
+    for (const { checkpoint, messages } of prefilled) {
+      this.prefilledMessages[checkpoint.number - 1] = messages;
+    }
+
+    // Register the Inbox buckets the streaming world-state synchronizer reconstructs each block's consumed
+    // message bundle from (AZIP-22 Fast Inbox): a genesis sentinel (totalMsgCount 0) so a leaf count of 0
+    // resolves to a bucket, plus one bucket per message-carrying checkpoint whose cumulative totalMsgCount
+    // matches the block's post-insertion L1-to-L2 leaf count. Rebuilt from the full prefilled chain (not just
+    // this call's checkpoints) so a reorg re-prefill that replaces a suffix keeps the cumulative aligned.
+    // Without these the synchronizer derives an empty bundle and the reconstructed block state diverges.
+    this.setInboxBucket(
+      {
+        seq: 0n,
+        inboxRollingHash: Fr.ZERO,
+        totalMsgCount: 0n,
+        timestamp: 0n,
+        msgCount: 0,
+        lastMessageIndex: 0n,
+        isOpen: false,
+      },
+      [],
+    );
+    let bucketSeq = 0n;
+    let totalMsgCount = 0n;
+    for (let i = 0; i < this.prefilled.length; i++) {
+      const messages = this.prefilledMessages[i] ?? [];
+      if (messages.length === 0) {
+        continue;
+      }
+      bucketSeq += 1n;
+      totalMsgCount += BigInt(messages.length);
+      this.setInboxBucket(
+        {
+          seq: bucketSeq,
+          inboxRollingHash: Fr.ZERO,
+          totalMsgCount,
+          timestamp: bucketSeq,
+          msgCount: messages.length,
+          lastMessageIndex: totalMsgCount - 1n,
+          isOpen: false,
+        },
+        messages,
+      );
+    }
   }
 
   public override createBlocks(numBlocks: number) {