test: add fetch mock helper and reaction coverage

src/browser/server-context.ensure-tab-available.prefers-last-target.test.ts

@@ -1,4 +1,5 @@
 import { describe, expect, it, vi } from "vitest";
+import { withFetchPreconnect } from "../test-utils/fetch-mock.js";
 import type { BrowserServerState } from "./server-context.js";
 import "./server-context.chrome-test-harness.js";
 import { createBrowserRouteContext } from "./server-context.js";
@@ -56,7 +57,7 @@ function stubChromeJsonList(responses: unknown[]) {
     } as unknown as Response;
   });
 
-  global.fetch = fetchMock;
+  global.fetch = withFetchPreconnect(fetchMock);
   return fetchMock;
 }