fix(telegram): ignore message_thread_id for non-forum group sessions

Regular Telegram groups (without Topics/Forums enabled) can send
message_thread_id when users reply to messages. This was incorrectly
being used to create separate session keys like '-123:topic:42',
causing each reply chain to get its own conversation context.

Now resolveTelegramForumThreadId only returns a thread ID when the
chat is actually a forum (is_forum=true). For regular groups, the
thread ID is ignored, ensuring all messages share the same session.

DMs continue to use messageThreadId for thread sessions as before.

src/telegram/bot.ts

@@ -427,7 +427,8 @@ export function createTelegramBot(opts: TelegramBotOptions) {
         peer: { kind: isGroup ? "group" : "dm", id: peerId },
       });
       const baseSessionKey = route.sessionKey;
-      const dmThreadId = !isGroup ? resolvedThreadId : undefined;
+      // DMs: use raw messageThreadId for thread sessions (not resolvedThreadId which is for forums)
+      const dmThreadId = !isGroup ? messageThreadId : undefined;
       const threadKeys =
         dmThreadId != null
           ? resolveThreadSessionKeys({ baseSessionKey, threadId: String(dmThreadId) })