fix(pxe): warn when block header unavailable for proven/finalized events (backport #22050) (#22547)

## Summary
Backport of #22050
to v4-next.

Cherry-pick applied cleanly. The noir-projects change from the original
PR was already present on v4-next, so only the PXE block_synchronizer
changes were needed.

### Changes
- Adds warning logs in PXE block synchronizer when `getBlockHeader`
returns null for `chain-proven` and `chain-finalized` events, instead of
silently skipping the anchor update
- Adds a comment clarifying the local promise capture pattern in
`sync()`

ClaudeBox log: https://claudebox.work/s/017ec0a44d097055?run=1

Author: benesjan

yarn-project/pxe/src/block_synchronizer/block_synchronizer.ts

@@ -77,6 +77,8 @@ export class BlockSynchronizer implements L2BlockStreamEventHandler {
           const blockHeader = await this.node.getBlockHeader(BlockNumber(event.block.number));
           if (blockHeader) {
             await this.updateAnchorBlockHeader(blockHeader);
+          } else {
+            this.log.warn(`Block header not found for proven block ${event.block.number}, skipping anchor update`);
           }
         }
         break;
@@ -86,6 +88,8 @@ export class BlockSynchronizer implements L2BlockStreamEventHandler {
           const blockHeader = await this.node.getBlockHeader(BlockNumber(event.block.number));
           if (blockHeader) {
             await this.updateAnchorBlockHeader(blockHeader);
+          } else {
+            this.log.warn(`Block header not found for finalized block ${event.block.number}, skipping anchor update`);
           }
         }
         break;
@@ -152,6 +156,8 @@ export class BlockSynchronizer implements L2BlockStreamEventHandler {
     }
 
     this.log.debug(`Syncing PXE with the node`);
+    // Capture the promise locally so we always await the exact promise we created, even if this.isSyncing is modified
+    // between assignment and await (e.g. due to future refactors introducing a yield point).
     const isSyncing = this.doSync();
     this.isSyncing = isSyncing;
     try {