fix: replace mock.module with spyOn to stop cross-file test pollution

mock.module("image-downloader") in forgejo-data-fetcher.test.ts replaced
the module in Bun's registry globally for the entire process. When Bun's
non-deterministic file ordering ran this file BEFORE image-downloader.test.ts,
the image-downloader tests imported a stubbed module instead of the real one,
breaking all 18 tests. This was the root cause of the CI failures that
persisted through 5 previous fix attempts.

spyOn patches the live module export per-test and is auto-restored by Bun
after each test — no cross-file pollution regardless of execution order.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/forgejo-data-fetcher.test.ts

@@ -1,4 +1,4 @@
-import { describe, expect, it, jest, mock } from "bun:test";
+import { beforeEach, describe, expect, it, jest, spyOn } from "bun:test";
 import {
   fetchForgejoData,
   mapFileStatus,
@@ -8,11 +8,12 @@ import {
 } from "../src/forgejo/data/fetcher";
 import type { Octokits } from "../src/github/api/client";
 import type { GitHubReviewComment } from "../src/github/types";
-
-// Mock image downloader — returns empty map (no-op on Forgejo)
-mock.module("../src/github/utils/image-downloader", () => ({
-  downloadCommentImages: jest.fn().mockResolvedValue(new Map()),
-}));
+// Import the module namespace so spyOn can patch the export directly.
+// This avoids mock.module() which replaces the module in Bun's registry
+// globally for the entire process — poisoning other test files
+// (like image-downloader.test.ts) that import the same module.
+// spyOn is auto-restored after each test, so no cross-file pollution.
+import * as imageDownloaderModule from "../src/github/utils/image-downloader";
 
 // Helper to build a mock Octokits with REST methods
 function createMockOctokits({
@@ -125,6 +126,15 @@ const mockIssueRestResponse = {
 };
 
 describe("fetchForgejoData", () => {
+  // Stub downloadCommentImages per-test — returns empty Map (no-op on Forgejo).
+  // spyOn patches the live module export and is auto-restored after each test,
+  // unlike mock.module which persists for the entire Bun process.
+  beforeEach(() => {
+    spyOn(imageDownloaderModule, "downloadCommentImages").mockResolvedValue(
+      new Map(),
+    );
+  });
+
   describe("PR data fetching", () => {
     it("should fetch PR details, files, commits, comments, and reviews in parallel then per-review comments", async () => {
       const mockOctokits = createMockOctokits({