Fix migration failing when importing either issues or PRs but not the other (#8892)

Related to https://codeberg.org/Codeberg/Community/issues/1944

* Allowed the githubdownloaderv3 to know whether issues and, or PRs are requested to migrate
* Used this information to decide to filter for "/pulls/" or "/issues"
  * Or not to filter at all if issues == true && prs == true
* Added isolated test for the downloader and for the uploader
* Created a new test_repo in github.com/forgejo and set it up properly together with @Gusted
* Updated github_downloader_test with the new URLs and test data from the repo
* Recorded the API calls for local testing
* Added a minimal gitbucket test (which uses the github downloader under the hood)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8892
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: patdyn <patdyn@noreply.codeberg.org>
Co-committed-by: patdyn <patdyn@noreply.codeberg.org>

services/migrations/gitbucket_test.go

@@ -0,0 +1,24 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package migrations
+
+import (
+	"os"
+	"testing"
+
+	"forgejo.org/models/unittest"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestGitbucketDownloaderCreation(t *testing.T) {
+	token := os.Getenv("GITHUB_READ_TOKEN")
+	fixturePath := "./testdata/github/full_download"
+	server := unittest.NewMockWebServer(t, "https://api.github.com", fixturePath, false)
+	defer server.Close()
+
+	downloader := NewGitBucketDownloader(t.Context(), server.URL, "", "", token, "forgejo", "test_repo")
+	err := downloader.RefreshRate()
+	require.NoError(t, err)
+}