SyncFromBrokerStage: report actual corpus_size in NewTestcase events

[0xmuon]

Description

I investigated how Event::NewTestcase { corpus_size, etc} is produced in fuzzer/mod.rs and consumed by the broker in events/simple.rs (via update_corpus_size on the client stats). In that path, corpus_size is clearly treated as clients current corpus size, not as a placeholder.
also to find behavior of SyncFromBrokerStage, I built and ran a custom multiclient fuzzer (sync_broker_test, which uses SyncFromBrokerStage) with --cores 0,1,2. I then set corpus_size set to both 0 and state.corpus().count() and tested, by adding temporary logging behind SYNC_CORPUS_SIZE flag to see the exact values being sent during sync(logs not included in commit as not needed). While the minimal tests UI did not visibly break when corpus_size was 0, the logs showed that SyncFromBrokerStage was the only NewTestcase producer that did not report the real state.corpus().count().Therefore, SyncFromBrokerStage now sends the actual state.corpus().count() as corpus_size, so all NewTestcase events follow the same semantics and future monitoring does not need to special‑case a hardcoded 0.
I can provide that fuzzer if you need.

• Tested locally via custom fuzzer(I tried using nautilus_sync example in multiclient but during runtime I couldn't see any response,so shifted to creating a different local fuzzer focused on using syncfrombrokerstage)

Checklist

• I have run ./scripts/precommit.sh and addressed all comments