Retry transient failures in star history generation (#12479)

* Initial plan

* Retry transient star history API failures

Co-authored-by: chrislusf <1543151+chrislusf@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: chrislusf <1543151+chrislusf@users.noreply.github.com>

.github/scripts/test_star_history.py

@@ -0,0 +1,75 @@
+import importlib.util
+import os
+from pathlib import Path
+import sys
+import types
+import unittest
+from unittest.mock import patch
+from urllib.error import HTTPError
+
+
+def load_star_history():
+    matplotlib = types.ModuleType("matplotlib")
+    matplotlib.use = lambda backend: None
+    pyplot = types.ModuleType("matplotlib.pyplot")
+    dates = types.ModuleType("matplotlib.dates")
+    dates.AutoDateLocator = object
+    dates.DateFormatter = object
+    modules = {
+        "matplotlib": matplotlib,
+        "matplotlib.pyplot": pyplot,
+        "matplotlib.dates": dates,
+    }
+    with patch.dict(sys.modules, modules), patch.dict(os.environ, {"GITHUB_TOKEN": "test"}):
+        path = Path(__file__).with_name("star_history.py")
+        spec = importlib.util.spec_from_file_location("star_history", path)
+        module = importlib.util.module_from_spec(spec)
+        spec.loader.exec_module(module)
+    return module
+
+
+star_history = load_star_history()
+
+
+class StarHistoryTests(unittest.TestCase):
+    def http_error(self, code):
+        return HTTPError("https://api.github.com", code, "error", {}, None)
+
+    @patch.object(star_history.time, "sleep")
+    @patch.object(star_history.urllib.request, "urlopen")
+    def test_retries_server_error(self, urlopen, sleep):
+        response = object()
+        urlopen.side_effect = [self.http_error(500), response]
+
+        result = star_history.open_url(object())
+
+        self.assertIs(result, response)
+        self.assertEqual(urlopen.call_count, 2)
+        sleep.assert_called_once_with(1)
+
+    @patch.object(star_history.time, "sleep")
+    @patch.object(star_history.urllib.request, "urlopen")
+    def test_does_not_retry_non_transient_http_error(self, urlopen, sleep):
+        error = self.http_error(404)
+        urlopen.side_effect = error
+
+        with self.assertRaises(HTTPError):
+            star_history.open_url(object())
+
+        urlopen.assert_called_once()
+        sleep.assert_not_called()
+
+    @patch.object(star_history.time, "sleep")
+    @patch.object(star_history.urllib.request, "urlopen")
+    def test_stops_after_retry_limit(self, urlopen, sleep):
+        urlopen.side_effect = self.http_error(500)
+
+        with self.assertRaises(HTTPError):
+            star_history.open_url(object())
+
+        self.assertEqual(urlopen.call_count, star_history.REQUEST_ATTEMPTS)
+        self.assertEqual(sleep.call_count, star_history.REQUEST_ATTEMPTS - 1)
+
+
+if __name__ == "__main__":
+    unittest.main()