fix(migrations): cancel GitLab version probes (#39023) (#39035)

Backport #39023 by @bircni

Bind the GitLab version probe to the migration context so a cancelled
migration does not remain blocked on a remote response.

Co-authored-by: bircni <bircni@icloud.com>

services/migrations/gitlab_test.go

@@ -4,6 +4,7 @@
 package migrations
 
 import (
+	"context"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -359,6 +360,28 @@ func TestGitlabDownloadRepo(t *testing.T) {
 	}, rvs)
 }
 
+func TestGitlabVersionProbeUsesMigrationContext(t *testing.T) {
+	started := make(chan struct{})
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		close(started)
+		<-r.Context().Done()
+	}))
+	defer server.Close()
+
+	ctx, cancel := context.WithCancel(t.Context())
+	defer cancel()
+
+	result := make(chan error, 1)
+	go func() {
+		_, err := NewGitlabDownloader(ctx, server.URL, "owner/repo", "")
+		result <- err
+	}()
+
+	<-started
+	cancel()
+	assert.Error(t, <-result)
+}
+
 func gitlabClientMockSetup(t *testing.T) (*http.ServeMux, *httptest.Server, *gitlab.Client) {
 	// mux is the HTTP request multiplexer used with the test server.
 	mux := http.NewServeMux()