test(stacks): drain pending download-metric records between tests (#1293)

The "records the download metric exactly once" test in stack-files-routes
intermittently failed with "expected 2 to be 1". The download route records
its metric asynchronously, on the file stream's end/close and the response's
close event, so a prior download test's record could land after the next test
called resetFileExplorerMetrics(), leaking a count across the reset. The
route's per-request recording is correct (a synchronous guard); the leak was
purely cross-test in the suite.

Add an afterEach in the download suite that waits for the download count to
settle (bounded poll) before the next test runs, so no pending record survives
the reset, and assert the count actually settled so a runaway record surfaces
rather than being silently swallowed.

backend/src/__tests__/stack-files-routes.test.ts

@@ -15,7 +15,7 @@
  * Community-tier success, input validation, upload size limit, and happy-path
  * 204/200 responses.
  */
-import { describe, it, expect, beforeAll, afterAll, beforeEach, vi } from 'vitest';
+import { describe, it, expect, beforeAll, afterAll, beforeEach, afterEach, vi } from 'vitest';
 import request from 'supertest';
 import bcrypt from 'bcrypt';
 import { promises as fs } from 'fs';
@@ -406,6 +406,26 @@ describe('GET /api/stacks/:stackName/files/content', () => {
 // ── GET /:stackName/files/download ────────────────────────────────────────────
 
 describe('GET /api/stacks/:stackName/files/download', () => {
+  // The download metric is recorded asynchronously, on the file stream's
+  // end/close and the response's close event, so it can land after a test's
+  // request has already resolved. Drain any such pending record before the next
+  // test runs: a prior download must not leak a count across the next test's
+  // resetFileExplorerMetrics() and inflate its "exactly once" assertion. Poll
+  // until the download count holds steady across several reads (bounded).
+  afterEach(async () => {
+    let prev = -1;
+    let stableReads = 0;
+    for (let i = 0; i < 50 && stableReads < 3; i++) {
+      const current = (await getDownloadMetric())?.count ?? 0;
+      stableReads = current === prev ? stableReads + 1 : 0;
+      prev = current;
+      await new Promise<void>((resolve) => setTimeout(resolve, 10));
+    }
+    // If the count never settled, a record is firing repeatedly (a real bug);
+    // surface it here rather than letting the bound silently swallow it.
+    expect(stableReads).toBeGreaterThanOrEqual(3);
+  });
+
   class CloseBeforeEndStream extends Readable {
     public bytesRead: number;