Clean up the codebase by removing unused and deprecated unread count
functionality in `ConversationCache` and associated Redis key
generators. This follows the transition to sequence-based unread
calculation.
- Remove `IncrementUnread`, `ClearUnread`, `GetUnreadCountFromHash`,
`GetAllUnreadCounts`, and `GetTotalUnread` from `ConversationCache`.
- Remove `PrefixUnreadHash` and `PrefixUnreadTotal` constants and
`UnreadHashKey`/`UnreadTotalKey` helper functions.
- Remove unused `isValidContentType` helper in `MessageHandler`.