fix(mattermost): pass payload.replyToId as root_id for threaded replies (#27744)

Merged via squash.

Prepared head SHA: e029079872
Co-authored-by: hnykda <2741256+hnykda@users.noreply.github.com>
Co-authored-by: mukhtharcm <56378562+mukhtharcm@users.noreply.github.com>
Reviewed-by: @mukhtharcm

src/auto-reply/reply/reply-plumbing.test.ts

@@ -230,6 +230,46 @@ describe("applyReplyThreading auto-threading", () => {
     expect(result[0].replyToId).toBe("42");
     expect(result[0].replyToTag).toBe(true);
   });
+
+  it("resolves [[reply_to_current]] to currentMessageId when replyToMode is 'all'", () => {
+    // Mattermost-style scenario: agent responds with [[reply_to_current]] and replyToMode
+    // is "all". The tag should resolve to the inbound message id.
+    const result = applyReplyThreading({
+      payloads: [{ text: "[[reply_to_current]] some reply text" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-abc123");
+    expect(result[0].replyToTag).toBe(true);
+    expect(result[0].text).toBe("some reply text");
+  });
+
+  it("resolves [[reply_to:<id>]] to explicit id when replyToMode is 'all'", () => {
+    const result = applyReplyThreading({
+      payloads: [{ text: "[[reply_to:mm-post-xyz789]] threaded reply" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-xyz789");
+    expect(result[0].text).toBe("threaded reply");
+  });
+
+  it("sets replyToId via implicit threading when replyToMode is 'all'", () => {
+    // Even without explicit tags, replyToMode "all" should set replyToId
+    // to currentMessageId for threading.
+    const result = applyReplyThreading({
+      payloads: [{ text: "hello" }],
+      replyToMode: "all",
+      currentMessageId: "mm-post-abc123",
+    });
+
+    expect(result).toHaveLength(1);
+    expect(result[0].replyToId).toBe("mm-post-abc123");
+  });
 });
 
 const baseRun: SubagentRunRecord = {