fix(coding-agent): refresh title after extension session rename

closes #3686

packages/coding-agent/test/suite/regressions/3686-session-name-event.test.ts

@@ -0,0 +1,40 @@
+import { afterEach, describe, expect, it } from "vitest";
+import type { ExtensionAPI } from "../../../src/index.js";
+import { createHarness, type Harness } from "../harness.js";
+
+describe("regression #3686: session name changes emit an event", () => {
+	const harnesses: Harness[] = [];
+
+	afterEach(() => {
+		while (harnesses.length > 0) {
+			harnesses.pop()?.cleanup();
+		}
+	});
+
+	it("emits session_info_changed when AgentSession.setSessionName is called", async () => {
+		const harness = await createHarness();
+		harnesses.push(harness);
+
+		harness.session.setSessionName("hello world");
+
+		expect(harness.sessionManager.getSessionName()).toBe("hello world");
+		expect(harness.eventsOfType("session_info_changed").map((event) => event.name)).toEqual(["hello world"]);
+	});
+
+	it("emits session_info_changed when an extension calls pi.setSessionName", async () => {
+		let api: ExtensionAPI | undefined;
+		const harness = await createHarness({
+			extensionFactories: [
+				(pi) => {
+					api = pi;
+				},
+			],
+		});
+		harnesses.push(harness);
+
+		api?.setSessionName("from extension");
+
+		expect(harness.sessionManager.getSessionName()).toBe("from extension");
+		expect(harness.eventsOfType("session_info_changed").map((event) => event.name)).toEqual(["from extension"]);
+	});
+});