test: use portable file reads under Vitest

tests/security-boundaries.test.ts

@@ -1,3 +1,5 @@
+import { readFile } from "node:fs/promises";
+
 import { describe, expect, it } from "vitest";
 
 const protectedEntryPoints = [
@@ -11,16 +13,20 @@ const protectedEntryPoints = [
 describe("administrative security boundaries", () => {
   for (const path of protectedEntryPoints) {
     it(`${path} repeats server-side admin authorization`, async () => {
-      const source = await Bun.file(new URL(`../${path}`, import.meta.url)).text();
+      const source = await readFile(
+        new URL(`../${path}`, import.meta.url),
+        "utf8",
+      );
       expect(source).toContain("requireAdmin");
       expect(source).toMatch(/await requireAdmin\(\)/u);
     });
   }
 
   it("same-origin media checks visibility before falling back to admin auth", async () => {
-    const source = await Bun.file(
+    const source = await readFile(
       new URL("../app/media/[id]/route.ts", import.meta.url),
-    ).text();
+      "utf8",
+    );
     expect(source).toContain("getMediaVisibility");
     expect(source).toContain("getAdminSession");
     expect(source.indexOf("getMediaVisibility")).toBeLessThan(