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/helpers.test.ts

@@ -3,8 +3,30 @@ import {
   buildTelegramThreadParams,
   buildTypingThreadParams,
   normalizeForwardedContext,
+  resolveTelegramForumThreadId,
 } from "./helpers.js";
 
+describe("resolveTelegramForumThreadId", () => {
+  it("returns undefined for non-forum groups even with messageThreadId", () => {
+    // Reply threads in regular groups should not create separate sessions
+    expect(resolveTelegramForumThreadId({ isForum: false, messageThreadId: 42 })).toBeUndefined();
+  });
+
+  it("returns undefined for non-forum groups without messageThreadId", () => {
+    expect(resolveTelegramForumThreadId({ isForum: false, messageThreadId: undefined })).toBeUndefined();
+    expect(resolveTelegramForumThreadId({ isForum: undefined, messageThreadId: 99 })).toBeUndefined();
+  });
+
+  it("returns General topic (1) for forum groups without messageThreadId", () => {
+    expect(resolveTelegramForumThreadId({ isForum: true, messageThreadId: undefined })).toBe(1);
+    expect(resolveTelegramForumThreadId({ isForum: true, messageThreadId: null })).toBe(1);
+  });
+
+  it("returns the topic id for forum groups with messageThreadId", () => {
+    expect(resolveTelegramForumThreadId({ isForum: true, messageThreadId: 99 })).toBe(99);
+  });
+});
+
 describe("buildTelegramThreadParams", () => {
   it("omits General topic thread id for message sends", () => {
     expect(buildTelegramThreadParams(1)).toBeUndefined();

src/telegram/bot/helpers.ts

@@ -13,14 +13,25 @@ import type {
 
 const TELEGRAM_GENERAL_TOPIC_ID = 1;
 
+/**
+ * Resolve the thread ID for Telegram forum topics.
+ * For non-forum groups, returns undefined even if messageThreadId is present
+ * (reply threads in regular groups should not create separate sessions).
+ * For forum groups, returns the topic ID (or General topic ID=1 if unspecified).
+ */
 export function resolveTelegramForumThreadId(params: {
   isForum?: boolean;
   messageThreadId?: number | null;
 }) {
-  if (params.isForum && params.messageThreadId == null) {
+  // Non-forum groups: ignore message_thread_id (reply threads are not real topics)
+  if (!params.isForum) {
+    return undefined;
+  }
+  // Forum groups: use the topic ID, defaulting to General topic
+  if (params.messageThreadId == null) {
     return TELEGRAM_GENERAL_TOPIC_ID;
   }
-  return params.messageThreadId ?? undefined;
+  return params.messageThreadId;
 }
 
 /**