TUI: keep streamed text when final output is empty

Closes #747

src/tui/tui.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from "vitest";
+
+import { resolveFinalAssistantText } from "./tui.js";
+
+describe("resolveFinalAssistantText", () => {
+  it("falls back to streamed text when final text is empty", () => {
+    expect(
+      resolveFinalAssistantText({ finalText: "", streamedText: "Hello" }),
+    ).toBe("Hello");
+  });
+
+  it("prefers the final text when present", () => {
+    expect(
+      resolveFinalAssistantText({
+        finalText: "All done",
+        streamedText: "partial",
+      }),
+    ).toBe("All done");
+  });
+});