FIX: Preveint recurring automations from running before start_date (#26963)

Some combinations of start_date and frequency/interval values can cause a recurring automation rule to either trigger before its start_date or never trigger. Example repros:

- Configure a recurring automation with hourly recurrence and a start_date several days ahead. What this will do is make the automation start running hourly immediately even though the start_date is several days ahead.

-  Configure a recurring automation with a weekly recurrence and a start_date several weeks ahead. This will result in the automation never triggering even after the start_date.

These 2 scenarios share the same cause which is that the automation plugin doesn't use the start_date as the date for the first run and instead uses the frequency/interval values from the current time to calculate the first run date.

This PR fixes this bug by adding an explicit check for start_date and using it as the first run's date if it's ahead of the current time.

plugins/automation/lib/discourse_automation/triggers/recurring.rb

@@ -36,6 +36,11 @@ module DiscourseAutomation
         end
 
         start_date = Time.parse(start_date)
+        if start_date > Time.zone.now
+          automation.pending_automations.create!(execute_at: start_date)
+          return
+        end
+
         byday = start_date.strftime("%A").upcase[0, 2]
         interval = interval.to_i
         interval_end = interval + 1