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

lib/exception_list.rb

@@ -16,4 +16,10 @@ module ExceptionList
     Errno::ECONNRESET, Errno::ENETUNREACH, Net::IMAP::ByeResponseError,
     SocketError
   ].freeze
+
+  IMAP_TRANSIENT_EXCEPTIONS = (IMAP_EXCEPTIONS + [
+    EOFError, OpenSSL::SSL::SSLError, Net::IMAP::NoResponseError, Net::IMAP::BadResponseError,
+    Net::IMAP::InvalidResponseError, Net::IMAP::ResponseParseError,
+    Net::IMAP::ResponseReadError, Net::IMAP::ResponseTooLargeError
+  ]).freeze
 end

lib/redis/redis_keys.rb

@@ -52,4 +52,8 @@ module Redis::RedisKeys
 
   ## Account Email Rate Limiting
   ACCOUNT_OUTBOUND_EMAIL_COUNT_KEY = 'OUTBOUND_EMAIL_COUNT::%<account_id>d::%<date>s'.freeze
+
+  ## Transient Error Backoff
+  BACKOFF_RETRY_COUNT = 'BACKOFF:%<obj_type>s:%<obj_id>d:retry_count'.freeze
+  BACKOFF_RETRY_AFTER = 'BACKOFF:%<obj_type>s:%<obj_id>d:retry_after'.freeze
 end