fix: complete extension usage accounting

closes #6509

packages/coding-agent/test/compaction-summary-reasoning.test.ts

@@ -1,7 +1,12 @@
 import type { AgentMessage } from "@earendil-works/pi-agent-core";
 import type { AssistantMessage, Model } from "@earendil-works/pi-ai";
 import { beforeEach, describe, expect, it, vi } from "vitest";
-import { type CompactionPreparation, compact, generateSummary } from "../src/core/compaction/index.ts";
+import {
+	type CompactionPreparation,
+	compact,
+	generateSummary,
+	generateSummaryWithUsage,
+} from "../src/core/compaction/index.ts";
 
 const { completeSimpleMock } = vi.hoisted(() => ({
 	completeSimpleMock: vi.fn(),
@@ -57,7 +62,7 @@ describe("generateSummary reasoning options", () => {
 	});
 
 	it("uses the provided thinking level for reasoning-capable models", async () => {
-		const result = await generateSummary(
+		const result = await generateSummaryWithUsage(
 			messages,
 			createModel(true),
 			2000,
@@ -79,6 +84,12 @@ describe("generateSummary reasoning options", () => {
 		});
 	});
 
+	it("preserves the string result from generateSummary", async () => {
+		await expect(generateSummary(messages, createModel(false), 2000, "test-key")).resolves.toBe(
+			"## Goal\nTest summary",
+		);
+	});
+
 	it("does not set reasoning when thinking is off", async () => {
 		await generateSummary(
 			messages,