Fix MySQL 8.2 TRUNCATE failures by correcting connection pool handling

The ReferentialIntegrity helper for MySQL had a bug where db.disconnect
was called before setting FOREIGN_KEY_CHECKS = 0. This caused the SET
command to execute on a different connection than the subsequent TRUNCATE
operations, leading to FK constraint violations.

This bug was exposed more consistently in CI with MySQL 8.2 after adding
indexes to the usage_events tables. The fix removes the db.disconnect
call and properly saves/restores the original FK checks value.

Fixes TRUNCATE errors in:
- spec/request/service_usage_events_spec.rb
- spec/request/app_usage_events_spec.rb
- All tests using isolation: :truncation with MySQL

Author: joyvuu-dave

spec/support/referential_integrity.rb

@@ -25,10 +25,13 @@ def without_referential_integrity_postgres
   end
 
   def without_referential_integrity_mysql
-    db.disconnect
+    # Save the current foreign_key_checks value and disable it
+    # Do NOT call db.disconnect as it would cause the SET command to execute
+    # on a different connection than the subsequent TRUNCATE operations
+    original_value = db.fetch('SELECT @@foreign_key_checks AS fk').first[:fk]
     db.run('SET FOREIGN_KEY_CHECKS = 0;')
     yield
   ensure
-    db.run('SET FOREIGN_KEY_CHECKS = 1;')
+    db.run("SET FOREIGN_KEY_CHECKS = #{original_value};")
   end
 end