fix(pxe): stop block synchronizer on PXE shutdown

[benesjan]

Summary

Fixes https://linear.app/aztec-labs/issue/F-339

The issue reported three resource leaks in PXE.stop():

1. L2BlockStream — active RunningPromise with timers: The issue assumed the block stream runs in polling mode (start()), but in the PXE context blockStream.start() is never called — the stream is only used in manual sync mode via blockStream.sync(). So there are no active timers or InterruptibleSleep handles keeping the event loop alive. That said, calling blockStream.stop() is still good defensive practice in case the usage mode changes.

2. LMDB store — open file descriptors: PXE.stop() already calls this.db.close(), so this part was already fixed at some point after the issue was filed.

3. HTTP keep-alive sockets from createAztecNodeClient: Node's built-in fetch (undici) uses HTTP keep-alive by default — after a request completes, the underlying TCP socket stays open in case another request comes to the same host. These idle sockets register as active handles on the event loop, preventing process.exit() from happening naturally. Fixing this would require either using a custom undici Agent/Pool with a close() method, or setting keepalive: false (which hurts performance). In practice this doesn't matter: the sockets time out on their own after a few seconds, and for long-running services (the main use case) the process stays up anyway. Only relevant if you need the process to exit immediately after stop() without calling process.exit(0).

What this PR actually fixes: BlockSynchronizer had no stop() method at all. If PXE.stop() were called while a sync was in progress, the ongoing sync could race against db.close(). While the job queue draining makes this unlikely in practice, adding an explicit stop() is the right defensive pattern and matches what ServerWorldStateSynchronizer already does.

Changes

• Adds a stop() method to BlockSynchronizer that awaits any in-progress sync, then stops the block stream
• Calls blockStateSynchronizer.stop() in PXE.stop() between draining the job queue and closing the DB
• Shutdown order: job queue drains (no new syncs) → synchronizer stops (waits for in-flight sync) → DB closes