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/jobs/inboxes/fetch_imap_emails_job_spec.rb

@@ -38,6 +38,14 @@ RSpec.describe Inboxes::FetchImapEmailsJob do
       end
     end
 
+    context 'when channel is in backoff' do
+      it 'does not fetch emails' do
+        allow(imap_email_channel).to receive(:in_backoff?).and_return(true)
+        expect(Imap::FetchEmailService).not_to receive(:new)
+        described_class.perform_now(imap_email_channel)
+      end
+    end
+
     context 'when the channel is regular imap' do
       it 'calls the imap fetch service' do
         fetch_service = double
@@ -56,6 +64,17 @@ RSpec.describe Inboxes::FetchImapEmailsJob do
         described_class.perform_now(imap_email_channel, 4)
         expect(fetch_service).to have_received(:perform)
       end
+
+      it 'clears backoff after successful fetch' do
+        fetch_service = double
+        allow(Imap::FetchEmailService).to receive(:new).and_return(fetch_service)
+        allow(fetch_service).to receive(:perform).and_return([])
+        allow(imap_email_channel).to receive(:clear_backoff!)
+
+        described_class.perform_now(imap_email_channel)
+
+        expect(imap_email_channel).to have_received(:clear_backoff!)
+      end
     end
 
     context 'when the channel is Microsoft' do
@@ -69,6 +88,37 @@ RSpec.describe Inboxes::FetchImapEmailsJob do
       end
     end
 
+    context 'when authentication error is raised' do
+      it 'calls authorization_error! on the channel' do
+        allow(Imap::FetchEmailService).to receive(:new).and_raise(Imap::AuthenticationError)
+        allow(imap_email_channel).to receive(:authorization_error!)
+
+        described_class.perform_now(imap_email_channel)
+
+        expect(imap_email_channel).to have_received(:authorization_error!)
+      end
+    end
+
+    context 'when a transient IMAP error is raised' do
+      it 'calls apply_backoff! on the channel' do
+        allow(Imap::FetchEmailService).to receive(:new).and_raise(EOFError)
+        allow(imap_email_channel).to receive(:apply_backoff!)
+
+        described_class.perform_now(imap_email_channel)
+
+        expect(imap_email_channel).to have_received(:apply_backoff!)
+      end
+    end
+
+    context 'when lock acquisition fails' do
+      it 'does not raise an error' do
+        lock_manager = instance_double(Redis::LockManager, lock: false)
+        allow(Redis::LockManager).to receive(:new).and_return(lock_manager)
+
+        expect { described_class.perform_now(imap_email_channel) }.not_to raise_error
+      end
+    end
+
     context 'when IMAP OAuth errors out' do
       it 'marks the connection as requiring authorization' do
         error_response = double