refactor(fast-inbox): TXE pads L1-to-L2 bundle only for first-in-checkpoint blocks (A-1380)

Align the TXE world-state caller with the per-block message-insertion semantics of
handleL2BlockAndMessages: pad the bundle only when the block is first in its checkpoint,
matching native world state. TXE mines one block per checkpoint (always the first block),
so this is a no-op for current behaviour but keeps the caller consistent with the new
per-block shape.

Author: spalladino

yarn-project/txe/src/state_machine/synchronizer.ts

@@ -35,10 +35,14 @@ export class TXESynchronizer implements WorldStateSynchronizer {
   }
 
   public async handleL2Block(block: L2Block, l1ToL2Messages: Fr[] = []) {
-    await this.nativeWorldStateService.handleL2BlockAndMessages(
-      block,
-      padArrayEnd<Fr, number>(l1ToL2Messages, Fr.ZERO, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP),
-    );
+    // Pad the bundle only for a first-in-checkpoint block, matching how the circuits (and native world state) build the
+    // message tree. TXE mines one block per checkpoint, so this is always the first block, but keep the condition
+    // explicit so the caller matches the per-block message-insertion semantics of handleL2BlockAndMessages.
+    const messages =
+      block.indexWithinCheckpoint === 0
+        ? padArrayEnd<Fr, number>(l1ToL2Messages, Fr.ZERO, NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP)
+        : l1ToL2Messages;
+    await this.nativeWorldStateService.handleL2BlockAndMessages(block, messages);
 
     this.blockNumber = block.header.globalVariables.blockNumber;
   }