feat: prevent deleted email conversations from syncing again (#14612)

# Pull Request Template

## Description

Prevent deleted email conversations from being synced into Chatwoot
again while they are still within the IMAP sync window.

When an admin explicitly deletes an email conversation, the incoming
email message IDs are stored temporarily in Redis. IMAP sync checks
these recently deleted message IDs in addition to existing message
records. Each Redis key expires automatically after two days.

This applies only to explicit conversation deletion. Individual message
deletion, inbox deletion, and account deletion keep their existing
behavior.

Fixes
[CW-7214](https://linear.app/chatwoot/issue/CW-7214/deleted-mails-in-gmail-inbox-gets-synced-again)

lib/redis/redis_keys.rb

@@ -2,6 +2,8 @@ module Redis::RedisKeys
   ## Inbox Keys
   # Array storing the ordered ids for agent round robin assignment
   ROUND_ROBIN_AGENTS = 'ROUND_ROBIN_AGENTS:%<inbox_id>d'.freeze
+  # Track recently deleted IMAP messages to prevent them from being synced again
+  IMAP_DELETED_MESSAGE = 'IMAP_DELETED_MESSAGE::%<inbox_id>d::%<message_id_digest>s'.freeze
 
   ## Conversation keys
   # Detect whether to send an email reply to the conversation