fix(test): SetRetryBackoff with correct slice length

Pass 2 elements to SetRetryBackoff (matching maxRetryAttempts-1 = 2)
and check the error return. Previously passing 1 element silently
failed, causing tests to fall back to default {1s, 2s} backoffs.

Fixes self-review finding: 429Retry tests now run in <10ms instead
of ~1s.

github/files_test.go

@@ -110,7 +110,9 @@ func TestGetFileContent_429Retry(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	content, err := c.GetFileContent(context.Background(), "owner", "repo", "file.go", "")
 	if err != nil {
@@ -228,7 +230,9 @@ func TestListContents_429Retry(t *testing.T) {
 
 	c := NewClient("token", srv.URL, AllowInsecureHTTP())
 	c.SetHTTPClient(srv.Client())
-	c.SetRetryBackoff([]time.Duration{1 * time.Millisecond})
+	if err := c.SetRetryBackoff([]time.Duration{1 * time.Millisecond, 1 * time.Millisecond}); err != nil {
+		t.Fatalf("SetRetryBackoff: %v", err)
+	}
 
 	entries, err := c.ListContents(context.Background(), "owner", "repo", "src")
 	if err != nil {