Return the context's error when it ends during a fetch request (closes #221)

retryingClient.get checked its context right after each request and
returned that request's error, so a deadline that passed while a 503 was
on its way came back as the 503. TestGetStopsWaitingWhenCanceled failed
on that now and then on a loaded host. When the context ends before a
retry, during the request or during the wait, get now returns an error
wrapping both the context's error and the last try's error.

The new test answers through a transport that holds the 503 until the
context has ended. A real server cannot do that: the client drops a
response still on its way when the context ends.

Model: opus-5-5

internal/cli/fetch_test.go

@@ -897,31 +897,23 @@ func TestGetHonorsRetryAfter(t *testing.T) {
 	}
 }
 
-// instant503Transport answers every request with a 503 at once, so the
+// TestGetStopsWaitingWhenCanceled gives get a server that always answers
-// 503 is in hand before any deadline can pass.
+// 503 and an hour's wait before each retry, then lets the context expire
-type instant503Transport struct{}
+// during that wait. get returning at all shows it stopped waiting.
-
-func (instant503Transport) RoundTrip(*http.Request) (*http.Response, error) {
-	return &http.Response{
-		StatusCode: http.StatusServiceUnavailable,
-		Body:       http.NoBody,
-	}, nil
-}
-
-// TestGetStopsWaitingWhenCanceled answers every request with a 503 and
-// sets an hour's wait before each retry, then lets the context expire
-// during that wait. get returning at all shows it stopped waiting; it must
-// return the deadline error and keep the 503 with it.
 func TestGetStopsWaitingWhenCanceled(t *testing.T) {
 	t.Parallel()
 
+	server := httptest.NewServer(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusServiceUnavailable)
+		}))
+	defer server.Close()
+
 	client := testClient()
-	client.client.Transport = instant503Transport{}
 	client.firstDelay = time.Hour
 
-	err := getNothing(client, "http://example.invalid/", 50*time.Millisecond)
+	err := getNothing(client, server.URL, 50*time.Millisecond)
 	require.ErrorIs(t, err, context.DeadlineExceeded)
-	require.ErrorIs(t, err, errHTTPStatus)
 }
 
 // late503Transport answers every request with a 503, but only once the