Move up creation of Rack instrumentation event (#1339)

We see a small difference between the transaction duration and the total
duration of child span groups.

Move up the transaction's Rack event creation so it happens directly
after the transaction is created to minimize this difference.

Context:
https://appsignal.slack.com/archives/C7XHXQ7N0/p1733142713081549?thread_ts=1733126790.603229&cid=C7XHXQ7N0

.changesets/reduce-rack-transaction-event-creation.md

@@ -0,0 +1,6 @@
+---
+bump: patch
+type: change
+---
+
+Minimize difference between Rack transaction duration and total child event durations.

lib/appsignal/rack/event_handler.rb

@@ -59,6 +59,7 @@ def on_start(request, _response)
           return unless request_handler?(request.env[APPSIGNAL_EVENT_HANDLER_ID])
 
           transaction = Appsignal::Transaction.create(Appsignal::Transaction::HTTP_REQUEST)
+          transaction.start_event
           request.env[APPSIGNAL_TRANSACTION] = transaction
 
           request.env[RACK_AFTER_REPLY] ||= []
@@ -83,7 +84,6 @@ def on_start(request, _response)
             # One such scenario is when a Puma "lowlevel_error" occurs.
             Appsignal::Transaction.complete_current!
           end
-          transaction.start_event
         end
       end