Two sites built ClientNotification messages with sprintf into a
fixed-size proto buffer with no length cap. The current format strings
fit comfortably, but a future caller editing either format string
without rechecking the buffer size would get a silent stack/heap
overrun. Switch to snprintf with sizeof so the bound is enforced at
the call site.
Co-authored-by: Ben Meadors <benmmeadors@gmail.com>
