feat(agent): add assistant model selector

tests/e2e/assistant.spec.ts

@@ -15,6 +15,7 @@ test("assistant chat smoke after opt-in", async ({ page }) => {
   await page.goto("/");
   await page.getByRole("button", { name: "Open assistant" }).click();
   await expect(page.getByRole("dialog", { name: "Assistant" })).toBeVisible();
+  await expect(page.getByRole("combobox", { name: "Assistant model" })).toBeVisible();
 
   await page.getByLabel("Message for Assistant").fill("hello assistant");
   await page.getByRole("button", { name: "Send" }).click();