fix: data races in Square mock and scheduler hostname tests

- square_dev.go: return a copy of CheckoutResult to the caller
  instead of sharing the map-stored pointer, preventing the goroutine
  from racing with the caller on Status field reads
- scheduler_test.go: remove t.Parallel from TestInitHostname_* tests
  which modify a shared package-level hostname variable

backend/internal/jobs/scheduler_test.go

@@ -554,8 +554,6 @@ func TestHostname_NonEmpty(t *testing.T) {
 // TestInitHostname_HappyPath verifies initHostname assigns the value from
 // the hostname resolver when it returns a valid name.
 func TestInitHostname_HappyPath(t *testing.T) {
-	t.Parallel()
-
 	orig := hostname
 	defer func() { hostname = orig }()
 
@@ -568,8 +566,6 @@ func TestInitHostname_HappyPath(t *testing.T) {
 // TestInitHostname_FallbackOnError verifies initHostname falls back to
 // "localhost" when the resolver returns an error.
 func TestInitHostname_FallbackOnError(t *testing.T) {
-	t.Parallel()
-
 	orig := hostname
 	defer func() { hostname = orig }()
 
@@ -582,8 +578,6 @@ func TestInitHostname_FallbackOnError(t *testing.T) {
 // TestInitHostname_FallbackOnEmpty verifies initHostname falls back to
 // "localhost" when the resolver returns an empty string (no error).
 func TestInitHostname_FallbackOnEmpty(t *testing.T) {
-	t.Parallel()
-
 	orig := hostname
 	defer func() { hostname = orig }()