This change updates how SMTP notifications are polled during scale tests.

Before, each of the ~2,000 pollers created its own http.Client, which opened thousands of short-lived TCP connections.
Under heavy load, this ran out of available network ports and caused errors likeconnect: cannot assign requested address

Now, all pollers share one HTTP connection pool. This prevents port exhaustion and makes polling faster and more stable.
If a network error happens, the poller will now retry instead of stopping, so tests keep running until all notifications are received.

TheSMTPRequestTimeout is now applied per request using a context, instead of being set on thehttp.Client.