fix(channels): add display_name to session metadata for Feishu, Zalo, Slack (#536)

Sessions page shows raw IDs instead of sender names because these channels
didn't include display_name in their message metadata. The gateway's
extractSessionMetadata() already looks for this key but it was missing
from Feishu, Zalo Personal, and Slack message payloads.

Co-authored-by: Luvu182 <208665161+Luvu182@users.noreply.github.com>

internal/channels/feishu/bot.go

@@ -169,6 +169,7 @@ func (c *Channel) handleMessageEvent(ctx context.Context, event *MessageEvent) {
 		"message_id":    messageID,
 		"chat_type":     mc.ChatType,
 		"sender_name":   senderName,
+		"display_name":  senderName,
 		"mentioned_bot": fmt.Sprintf("%t", mc.MentionedBot),
 		"platform":      channels.TypeFeishu,
 	}