fix(mattermost): read replyTo param in plugin handleAction send (#41176)

Merged via squash.

Prepared head SHA: 33cac4c33f
Co-authored-by: hnykda <2741256+hnykda@users.noreply.github.com>
Co-authored-by: mukhtharcm <56378562+mukhtharcm@users.noreply.github.com>
Reviewed-by: @mukhtharcm

extensions/mattermost/src/channel.test.ts

@@ -214,6 +214,57 @@ describe("mattermostPlugin", () => {
       ]);
       expect(result?.details).toEqual({});
     });
+
+    it("maps replyTo to replyToId for send actions", async () => {
+      const cfg = createMattermostTestConfig();
+
+      await mattermostPlugin.actions?.handleAction?.({
+        channel: "mattermost",
+        action: "send",
+        params: {
+          to: "channel:CHAN1",
+          message: "hello",
+          replyTo: "post-root",
+        },
+        cfg,
+        accountId: "default",
+      } as any);
+
+      expect(sendMessageMattermostMock).toHaveBeenCalledWith(
+        "channel:CHAN1",
+        "hello",
+        expect.objectContaining({
+          accountId: "default",
+          replyToId: "post-root",
+        }),
+      );
+    });
+
+    it("falls back to trimmed replyTo when replyToId is blank", async () => {
+      const cfg = createMattermostTestConfig();
+
+      await mattermostPlugin.actions?.handleAction?.({
+        channel: "mattermost",
+        action: "send",
+        params: {
+          to: "channel:CHAN1",
+          message: "hello",
+          replyToId: "   ",
+          replyTo: " post-root ",
+        },
+        cfg,
+        accountId: "default",
+      } as any);
+
+      expect(sendMessageMattermostMock).toHaveBeenCalledWith(
+        "channel:CHAN1",
+        "hello",
+        expect.objectContaining({
+          accountId: "default",
+          replyToId: "post-root",
+        }),
+      );
+    });
   });
 
   describe("outbound", () => {

extensions/mattermost/src/channel.ts

@@ -157,7 +157,9 @@ const mattermostMessageActions: ChannelMessageActionAdapter = {
     }
 
     const message = typeof params.message === "string" ? params.message : "";
-    const replyToId = typeof params.replyToId === "string" ? params.replyToId : undefined;
+    // Match the shared runner semantics: trim empty reply IDs away before
+    // falling back from replyToId to replyTo on direct plugin calls.
+    const replyToId = readMattermostReplyToId(params);
     const resolvedAccountId = accountId || undefined;
 
     const mediaUrl =
@@ -201,6 +203,18 @@ const meta = {
   quickstartAllowFrom: true,
 } as const;
 
+function readMattermostReplyToId(params: Record<string, unknown>): string | undefined {
+  const readNormalizedValue = (value: unknown) => {
+    if (typeof value !== "string") {
+      return undefined;
+    }
+    const trimmed = value.trim();
+    return trimmed || undefined;
+  };
+
+  return readNormalizedValue(params.replyToId) ?? readNormalizedValue(params.replyTo);
+}
+
 function normalizeAllowEntry(entry: string): string {
   return entry
     .trim()