[BUG] `NeugDBService` never updates `running_`, causing false stop-state reporting

[d1zzy126]

Describe the bug
NeugDBService has a running_ atomic that is consulted by IsRunning(), service_status(), and Stop(), but that state is never updated on successful start or stop. As a result, Stop() can immediately print NeugDB service has not been started! right after a successful serve(), and the service's internal running-state reporting is inconsistent with reality.

Execution Logs
Minimal Python reproducer:

from neug.database import Database

db = Database("", "w")
db.serve(19001, "127.0.0.1", False)
db.stop_serving()
db.close()

This reproducer emits the incorrect stop-state message immediately after a successful serve().

Expected behavior
After a successful serve(), the service should be considered running. IsRunning() and service_status() should reflect that state, and stop_serving() should not claim the service was never started.

Error Message
Observed output:

NeugDB service has not been started!

Source inspection shows that running_ exists in the service object and is used by IsRunning(), service_status(), and Stop(), but Start() and Stop() never set or clear it, so IsRunning() remains false throughout the service lifetime.

[zhanglei1949]

Thanks for reporting the issue! We will fix it ASAP.