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-message-context.ts

@@ -173,7 +173,8 @@ export const buildTelegramMessageContext = async ({
     },
   });
   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) })
@@ -601,7 +602,8 @@ export const buildTelegramMessageContext = async ({
     Sticker: allMedia[0]?.stickerMetadata,
     ...(locationData ? toLocationContext(locationData) : undefined),
     CommandAuthorized: commandAuthorized,
-    MessageThreadId: resolvedThreadId,
+    // For groups: use resolvedThreadId (forum topics only); for DMs: use raw messageThreadId
+    MessageThreadId: isGroup ? resolvedThreadId : messageThreadId,
     IsForum: isForum,
     // Originating channel for reply routing.
     OriginatingChannel: "telegram" as const,