fix: Increase the parallelism config to fix flaky tests, revert bad commits (#13410)

The specs break only in Circle CI, we have to figure out the root cause
for the same. At the moment, I have increased the parallelism to fix
this.

db/migrate/20260130061021_add_index_to_reporting_events_for_response_distribution.rb

@@ -0,0 +1,11 @@
+class AddIndexToReportingEventsForResponseDistribution < ActiveRecord::Migration[7.1]
+  disable_ddl_transaction!
+
+  def change
+    add_index :reporting_events,
+              [:account_id, :name, :inbox_id, :created_at],
+              name: 'index_reporting_events_for_response_distribution',
+              algorithm: :concurrently,
+              if_not_exists: true
+  end
+end