fix: handle forum/topics in Telegram DM thread routing (#17980)

resolveTelegramThreadSpec now checks isForum in the non-group path.
DMs with forum/topics enabled return scope 'forum' so each topic
gets its own session, while plain DM threads keep scope 'dm'.

src/telegram/bot/helpers.test.ts

@@ -5,6 +5,7 @@ import {
   expandTextLinks,
   normalizeForwardedContext,
   resolveTelegramForumThreadId,
+  resolveTelegramThreadSpec,
 } from "./helpers.js";
 
 describe("resolveTelegramForumThreadId", () => {
@@ -32,6 +33,34 @@ describe("resolveTelegramForumThreadId", () => {
   });
 });
 
+describe("resolveTelegramThreadSpec", () => {
+  it("returns dm scope for plain DM (no forum, no thread id)", () => {
+    expect(resolveTelegramThreadSpec({ isGroup: false })).toEqual({ scope: "dm" });
+  });
+
+  it("preserves thread id with dm scope when DM has thread id but is not a forum", () => {
+    expect(
+      resolveTelegramThreadSpec({ isGroup: false, isForum: false, messageThreadId: 42 }),
+    ).toEqual({ id: 42, scope: "dm" });
+  });
+
+  it("returns forum scope when DM has isForum and thread id", () => {
+    expect(
+      resolveTelegramThreadSpec({ isGroup: false, isForum: true, messageThreadId: 99 }),
+    ).toEqual({ id: 99, scope: "forum" });
+  });
+
+  it("falls back to dm scope when DM has isForum but no thread id", () => {
+    expect(resolveTelegramThreadSpec({ isGroup: false, isForum: true })).toEqual({ scope: "dm" });
+  });
+
+  it("delegates to group path for groups", () => {
+    expect(
+      resolveTelegramThreadSpec({ isGroup: true, isForum: true, messageThreadId: 50 }),
+    ).toEqual({ id: 50, scope: "forum" });
+  });
+});
+
 describe("buildTelegramThreadParams", () => {
   it("omits General topic thread id for message sends", () => {
     expect(buildTelegramThreadParams({ id: 1, scope: "forum" })).toBeUndefined();

src/telegram/bot/helpers.ts

@@ -101,13 +101,15 @@ export function resolveTelegramThreadSpec(params: {
       scope: params.isForum ? "forum" : "none",
     };
   }
-  if (params.messageThreadId == null) {
+  // DM with forum/topics enabled — treat like a forum, not a flat DM
-    return { scope: "dm" };
+  if (params.isForum && params.messageThreadId != null) {
+    return { id: params.messageThreadId, scope: "forum" };
   }
-  return {
+  // Preserve thread ID for non-forum DM threads (session routing, #8891)
-    id: params.messageThreadId,
+  if (params.messageThreadId != null) {
-    scope: "dm",
+    return { id: params.messageThreadId, scope: "dm" };
-  };
+  }
+  return { scope: "dm" };
 }
 
 /**