fix: sync linked discussion guest support

internal/rpc/linked_discussion_projection.go

@@ -0,0 +1,65 @@
+package rpc
+
+import (
+	"context"
+
+	"github.com/gotd/td/tg"
+
+	"telesrv/internal/domain"
+)
+
+type linkedDiscussionChannelProvider interface {
+	GetLinkedDiscussionChannel(ctx context.Context, userID, sourceChannelID int64) (domain.ChannelView, error)
+}
+
+// linkedDiscussionChat projects the linked megagroup through the source
+// broadcast. Generic channel lookup must keep rejecting private non-members;
+// this narrow path is what lets TDesktop resolve ChannelFull.linked_chat_id.
+func (r *Router) linkedDiscussionChat(ctx context.Context, userID, sourceChannelID int64) (tg.ChatClass, bool) {
+	provider, ok := r.deps.Channels.(linkedDiscussionChannelProvider)
+	if !ok || userID == 0 || sourceChannelID == 0 {
+		return nil, false
+	}
+	view, err := provider.GetLinkedDiscussionChannel(ctx, userID, sourceChannelID)
+	if err != nil || view.Channel.ID == 0 {
+		return nil, false
+	}
+	return tgChannelChatForView(userID, view), true
+}
+
+func (r *Router) appendLinkedDiscussionChat(ctx context.Context, userID, sourceChannelID int64, chats []tg.ChatClass) []tg.ChatClass {
+	chat, ok := r.linkedDiscussionChat(ctx, userID, sourceChannelID)
+	if !ok {
+		return chats
+	}
+	return replaceTGChat(chats, chat)
+}
+
+func replaceTGChat(chats []tg.ChatClass, replacement tg.ChatClass) []tg.ChatClass {
+	wanted := tgChatID(replacement)
+	if wanted == 0 {
+		return chats
+	}
+	out := make([]tg.ChatClass, 0, len(chats)+1)
+	for _, chat := range chats {
+		if tgChatID(chat) != wanted {
+			out = append(out, chat)
+		}
+	}
+	return append(out, replacement)
+}
+
+func tgChatID(chat tg.ChatClass) int64 {
+	switch item := chat.(type) {
+	case *tg.Channel:
+		return item.ID
+	case *tg.ChannelForbidden:
+		return item.ID
+	case *tg.Chat:
+		return item.ID
+	case *tg.ChatForbidden:
+		return item.ID
+	default:
+		return 0
+	}
+}