Prevent silent failure logging when 4142 mailer is sent (#19432)

Ensures the veteran-facing failure mailer we send when a 4142 document fails to upload for a Form 526 is the last thing that can raise an exception in the upload job's sidekiq_retries_exhausted block.

app/sidekiq/central_mail/submit_form4142_job.rb

@@ -76,6 +76,11 @@ class CentralMailResponseError  < Common::Exceptions::BackendServiceException; e
       if Flipper.enabled?(:form526_send_4142_failure_notification)
         EVSS::DisabilityCompensationForm::Form4142DocumentUploadFailureEmail.perform_async(form526_submission_id)
       end
+      # NOTE: do NOT add any additional code here between the failure email being enqueued and the rescue block.
+      # The mailer prevents an upload from failing silently, since we notify the veteran and provide a workaround.
+      # The rescue will catch any errors in the sidekiq_retries_exhausted block and mark a "silent failure".
+      # This shouldn't happen if an email was sent; there should be no code here to throw an additional exception.
+      # The mailer should be the last thing that can fail.
     rescue => e
       cl = caller_locations.first
       call_location = Logging::CallLocation.new(ZSF_DD_TAG_FUNCTION, cl.path, cl.lineno)