fix: call authorization_error! on IMAP auth failures (#13560)

## Notion document

https://www.notion.so/chatwoot/Email-IMAP-Issue-30aa5f274c928062aa6bddc2e5877a63?showMoveTo=true&saveParent=true

## Description

PLAIN IMAP channels (non-OAuth) were silently retrying failed
authentication every minute, forever. When credentials are
wrong/expired, Net::IMAP::NoResponseError was caught and logged but
channel.authorization_error! was never called — so the Redis error
counter never incremented, reauthorization_required? was never set, and
admins were never notified. OAuth channels already had this handled
correctly via the Reauthorizable concern.
Additionally, Net::IMAP::ResponseParseError (raised by non-RFC-compliant
IMAP servers) was falling through to the StandardError catch-all,
flooding
Estimated impact before fix: ~70–75 broken IMAP inboxes generating
~700k–750k wasted Sidekiq jobs/week.

## Type of change

- [ ] Bug fix (non-breaking change which fixes an issue)

## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

spec/models/channel/email_spec.rb

@@ -2,12 +2,14 @@
 
 require 'rails_helper'
 require Rails.root.join 'spec/models/concerns/reauthorizable_shared.rb'
+require Rails.root.join 'spec/models/concerns/backoffable_shared.rb'
 
 RSpec.describe Channel::Email do
   let(:channel) { create(:channel_email) }
 
   describe 'concerns' do
     it_behaves_like 'reauthorizable'
+    it_behaves_like 'backoffable'
 
     context 'when prompt_reauthorization!' do
       it 'calls channel notifier mail for email' do