fix(download): 下载失败冷却日志补充失败原因和重试时间

将 _active_download_failure_fingerprints 返回类型从 Set[str] 改为 Dict[str, DownloadFailure]，
__is_context_in_failure_cooldown 从失败记录中提取 error_message 和 next_retry_at，
日志输出格式改为「近期添加下载失败（失败原因：xxx），暂时跳过该资源，将于 yyyy-MM-dd HH:mm:ss 后重试」。

tests/test_download_chain.py

@@ -921,7 +921,11 @@ def test_batch_download_skips_failed_subscription_resource_and_tries_next(monkey
             """
             assert now_time
             assert failed_fingerprint in fingerprints
-            return {failed_fingerprint: SimpleNamespace(fingerprint=failed_fingerprint)}
+            return {failed_fingerprint: SimpleNamespace(
+                fingerprint=failed_fingerprint,
+                error_message="无法读取种子文件",
+                next_retry_at="2026-01-02 03:04:05",
+            )}
 
     monkeypatch.setattr(download_module, "DownloadFailureOper", _ActiveDownloadFailureOper)