fix(gateway): preserve agentId through gateway send path

Landed from #23249 by @Sid-Qin.
Includes extra regression tests for agentId precedence + blank fallback.

Co-authored-by: Sid <201593046+Sid-Qin@users.noreply.github.com>

src/infra/outbound/message.channels.test.ts

@@ -194,6 +194,35 @@ describe("gateway url override hardening", () => {
       }),
     );
   });
+
+  it("forwards explicit agentId in gateway send params", async () => {
+    setRegistry(
+      createTestRegistry([
+        {
+          pluginId: "mattermost",
+          source: "test",
+          plugin: {
+            ...createMattermostLikePlugin({ onSendText: () => {} }),
+            outbound: { deliveryMode: "gateway" },
+          },
+        },
+      ]),
+    );
+
+    callGatewayMock.mockResolvedValueOnce({ messageId: "m-agent" });
+    await sendMessage({
+      cfg: {},
+      to: "channel:town-square",
+      content: "hi",
+      channel: "mattermost",
+      agentId: "work",
+    });
+
+    const call = callGatewayMock.mock.calls[0]?.[0] as {
+      params?: Record<string, unknown>;
+    };
+    expect(call.params?.agentId).toBe("work");
+  });
 });
 
 const emptyRegistry = createTestRegistry([]);

src/infra/outbound/message.ts

@@ -251,6 +251,7 @@ export async function sendMessage(params: MessageSendParams): Promise<MessageSen
       mediaUrls: mirrorMediaUrls.length ? mirrorMediaUrls : params.mediaUrls,
       gifPlayback: params.gifPlayback,
       accountId: params.accountId,
+      agentId: params.agentId,
       channel,
       sessionKey: params.mirror?.sessionKey,
       idempotencyKey: params.idempotencyKey ?? randomIdempotencyKey(),