Add unit tests for vercel-ai-stream extractUserText and stream listener

Tests cover extractUserText with v5+ parts format, v4 content string,
last-user-message extraction, simple text/prompt fields, null cases, and
preference ordering. Stream listener tests verify text event translation
and the writableEnded guard.

packages/coding-agent/test/vercel-ai-stream.test.ts

@@ -0,0 +1,132 @@
+import { describe, it, expect } from "vitest";
+import type { AgentSessionEvent } from "../src/core/agent-session.js";
+import { extractUserText, createVercelStreamListener, finishVercelStream } from "../src/core/vercel-ai-stream.js";
+
+describe("extractUserText", () => {
+	it("extracts text from useChat v5+ format with parts", () => {
+		const body = {
+			messages: [
+				{ role: "user", parts: [{ type: "text", text: "hello world" }] },
+			],
+		};
+		expect(extractUserText(body)).toBe("hello world");
+	});
+
+	it("extracts text from useChat v4 format with content string", () => {
+		const body = {
+			messages: [{ role: "user", content: "hello world" }],
+		};
+		expect(extractUserText(body)).toBe("hello world");
+	});
+
+	it("extracts last user message when multiple messages present", () => {
+		const body = {
+			messages: [
+				{ role: "user", parts: [{ type: "text", text: "first" }] },
+				{ role: "assistant", parts: [{ type: "text", text: "response" }] },
+				{ role: "user", parts: [{ type: "text", text: "second" }] },
+			],
+		};
+		expect(extractUserText(body)).toBe("second");
+	});
+
+	it("extracts text from simple gateway format", () => {
+		expect(extractUserText({ text: "hello" })).toBe("hello");
+	});
+
+	it("extracts text from prompt format", () => {
+		expect(extractUserText({ prompt: "hello" })).toBe("hello");
+	});
+
+	it("returns null for empty body", () => {
+		expect(extractUserText({})).toBeNull();
+	});
+
+	it("returns null for empty messages array", () => {
+		expect(extractUserText({ messages: [] })).toBeNull();
+	});
+
+	it("prefers text field over messages", () => {
+		const body = {
+			text: "direct",
+			messages: [{ role: "user", parts: [{ type: "text", text: "from messages" }] }],
+		};
+		expect(extractUserText(body)).toBe("direct");
+	});
+});
+
+describe("createVercelStreamListener", () => {
+	function createMockResponse() {
+		const chunks: string[] = [];
+		let ended = false;
+		return {
+			writableEnded: false,
+			write(data: string) {
+				chunks.push(data);
+				return true;
+			},
+			end() {
+				ended = true;
+				this.writableEnded = true;
+			},
+			chunks,
+			get ended() { return ended; },
+		} as any;
+	}
+
+	function parseChunks(chunks: string[]): Array<object | string> {
+		return chunks
+			.filter((c) => c.startsWith("data: "))
+			.map((c) => {
+				const payload = c.replace(/^data: /, "").replace(/\n\n$/, "");
+				try { return JSON.parse(payload); }
+				catch { return payload; }
+			});
+	}
+
+	it("translates text streaming events", () => {
+		const response = createMockResponse();
+		const listener = createVercelStreamListener(response);
+
+		listener({ type: "agent_start" } as AgentSessionEvent);
+		listener({ type: "turn_start", turnIndex: 0, timestamp: Date.now() } as AgentSessionEvent);
+		listener({
+			type: "message_update",
+			message: {} as any,
+			assistantMessageEvent: { type: "text_start", contentIndex: 0, partial: {} as any },
+		} as AgentSessionEvent);
+		listener({
+			type: "message_update",
+			message: {} as any,
+			assistantMessageEvent: { type: "text_delta", contentIndex: 0, delta: "hello", partial: {} as any },
+		} as AgentSessionEvent);
+		listener({
+			type: "message_update",
+			message: {} as any,
+			assistantMessageEvent: { type: "text_end", contentIndex: 0, content: "hello", partial: {} as any },
+		} as AgentSessionEvent);
+		listener({ type: "turn_end", turnIndex: 0, message: {} as any, toolResults: [] } as AgentSessionEvent);
+
+		const parsed = parseChunks(response.chunks);
+		expect(parsed).toEqual([
+			{ type: "start" },
+			{ type: "start-step" },
+			{ type: "text-start", id: "text_0" },
+			{ type: "text-delta", id: "text_0", delta: "hello" },
+			{ type: "text-end", id: "text_0" },
+			{ type: "finish-step" },
+		]);
+	});
+
+	it("does not write after response has ended", () => {
+		const response = createMockResponse();
+		const listener = createVercelStreamListener(response);
+
+		listener({ type: "agent_start" } as AgentSessionEvent);
+		response.end();
+		listener({ type: "turn_start", turnIndex: 0, timestamp: Date.now() } as AgentSessionEvent);
+
+		const parsed = parseChunks(response.chunks);
+		expect(parsed).toEqual([{ type: "start" }]);
+	});
+});