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/concerns/backoffable_shared.rb

@@ -0,0 +1,43 @@
+require 'rails_helper'
+
+shared_examples_for 'backoffable' do
+  let(:obj) { FactoryBot.create(described_class.to_s.underscore.tr('/', '_').to_sym) }
+
+  before do
+    allow(GlobalConfigService).to receive(:load).with('BACKOFF_MAX_INTERVAL_MINUTES', 5).and_return(2)
+    allow(GlobalConfigService).to receive(:load).with('BACKOFF_MAX_INTERVAL_COUNT', 10).and_return(3)
+    # max_interval=2, max_retries=(2-1)+3=4; exhausts on 5th apply_backoff!
+  end
+
+  it 'starts with no backoff' do
+    expect(obj.in_backoff?).to be false
+    expect(obj.backoff_retry_count).to eq 0
+  end
+
+  it 'ramps backoff on each failure' do
+    obj.apply_backoff!
+    expect(obj.backoff_retry_count).to eq 1
+    expect(obj.in_backoff?).to be true
+  end
+
+  it 'caps wait time at max interval' do
+    4.times { obj.apply_backoff! }
+    expect(obj.backoff_retry_count).to eq 4
+    expect(obj.in_backoff?).to be true
+  end
+
+  it 'exhausts backoff and calls prompt_reauthorization! after max retries' do
+    allow(obj).to receive(:prompt_reauthorization!)
+    5.times { obj.apply_backoff! }
+    expect(obj).to have_received(:prompt_reauthorization!)
+    expect(obj.backoff_retry_count).to eq 0
+    expect(obj.in_backoff?).to be false
+  end
+
+  it 'clear_backoff! resets retry count and backoff window' do
+    obj.apply_backoff!
+    obj.clear_backoff!
+    expect(obj.in_backoff?).to be false
+    expect(obj.backoff_retry_count).to eq 0
+  end
+end