refactor(cache): remove deprecated unread count methods and keys

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`.

internal/cache/keys.go

@@ -26,9 +26,7 @@ const (
 	PrefixUnreadSystem       = "unread:system"
 	PrefixUnreadConversation = "unread:conversation"
 	PrefixUnreadDetail       = "unread:detail"
-	PrefixUnreadHash         = "unread:hash"
-	PrefixUnreadTotal        = "unread:total"
-
+		
 	// 已读位置相关（OpenIM 风格：缓存 hasReadSeq，用于 O(1) 未读数计算）
 	PrefixUserReadSeq = "user_read_seq"
 
@@ -123,16 +121,6 @@ func UnreadDetailKey(userID, conversationID string) string {
 	return fmt.Sprintf("%s:%s:%s", PrefixUnreadDetail, userID, conversationID)
 }
 
-// UnreadHashKey 生成用户未读数 Hash 缓存键
-func UnreadHashKey(userID string) string {
-	return fmt.Sprintf("%s:%s", PrefixUnreadHash, userID)
-}
-
-// UnreadTotalKey 生成用户总未读数计数器键
-func UnreadTotalKey(userID string) string {
-	return fmt.Sprintf("%s:%s", PrefixUnreadTotal, userID)
-}
-
 // UserInfoKey 生成用户信息缓存键
 func UserInfoKey(userID string) string {
 	return fmt.Sprintf("%s:%s", PrefixUserInfo, userID)