feat: add test notification endpoint

[yashmehrotra]

Fixes: #920

[graphite-app]

The empty error blocks silently swallow errors that should be surfaced to the client. Consider handling both error cases by returning them like:

if err != nil {
    return dutyAPI.WriteError(c, dutyAPI.Errorf(dutyAPI.EINTERNAL, "failed to get env for event: %v", err))
}
if err := SendNotification(notificationContext, payload, celEnv); err != nil {
    return dutyAPI.WriteError(c, dutyAPI.Errorf(dutyAPI.EINTERNAL, "failed to send notification: %v", err))
}

This provides better visibility into failures and maintains consistent error handling patterns.

Spotted by Graphite Reviewer

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