test(e2e): fix flaky model switchto test race condition

The test previously expected `after.modelId` to equal `before.modelId`
after successfully switching to `gpt-4.1`. This only passed due to a
race condition where the `session.model_change` event hadn't yet
propagated to the client before `getCurrent()` was called. On some
environments (like Windows CI), the event arrived faster, causing the
model to correctly update to `gpt-4.1`, which then failed the logically
incorrect assertion.

This commit properly awaits the `session.model_change` event and
asserts that the model successfully changed to `gpt-4.1`.

Author: Morabbin

nodejs/test/e2e/rpc_session_state.e2e.test.ts

@@ -62,10 +62,11 @@ describe("Session-scoped RPC", async () => {
             modelId: "gpt-4.1",
             reasoningEffort: "high",
         });
+        await waitForEvent(session, (event) => event.type === "session.model_change" && event.data.newModel === "gpt-4.1", "session.model_change event after switchTo");
         const after = await session.rpc.model.getCurrent();
 
         expect(result.modelId).toBe("gpt-4.1");
-        expect(after.modelId).toBe(before.modelId);
+        expect(after.modelId).toBe("gpt-4.1");
 
         await session.disconnect();
     });