fix: refactor TUI stream assembly (#1202, thanks @aaronveklabs)

Co-authored-by: Aaron <aaron@vektor-labs.com>

src/tui/tui-stream-assembler.test.ts

@@ -0,0 +1,68 @@
+import { describe, expect, it } from "vitest";
+
+import { TuiStreamAssembler } from "./tui-stream-assembler.js";
+
+describe("TuiStreamAssembler", () => {
+  it("keeps thinking before content even when thinking arrives later", () => {
+    const assembler = new TuiStreamAssembler();
+    const first = assembler.ingestDelta(
+      "run-1",
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Hello" }],
+      },
+      true,
+    );
+    expect(first).toBe("Hello");
+
+    const second = assembler.ingestDelta(
+      "run-1",
+      {
+        role: "assistant",
+        content: [{ type: "thinking", thinking: "Brain" }],
+      },
+      true,
+    );
+    expect(second).toBe("[thinking]\nBrain\n\nHello");
+  });
+
+  it("omits thinking when showThinking is false", () => {
+    const assembler = new TuiStreamAssembler();
+    const text = assembler.ingestDelta(
+      "run-2",
+      {
+        role: "assistant",
+        content: [
+          { type: "thinking", thinking: "Hidden" },
+          { type: "text", text: "Visible" },
+        ],
+      },
+      false,
+    );
+
+    expect(text).toBe("Visible");
+  });
+
+  it("falls back to streamed text on empty final payload", () => {
+    const assembler = new TuiStreamAssembler();
+    assembler.ingestDelta(
+      "run-3",
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Streamed" }],
+      },
+      false,
+    );
+
+    const finalText = assembler.finalize(
+      "run-3",
+      {
+        role: "assistant",
+        content: [],
+      },
+      false,
+    );
+
+    expect(finalText).toBe("Streamed");
+  });
+});