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

@@ -916,6 +916,34 @@ func TestGetStopsWaitingWhenCanceled(t *testing.T) {
 	require.ErrorIs(t, err, context.DeadlineExceeded)
 }
 
+// late503Transport answers every request with a 503, but only once the
+// request's context has ended. A real server cannot stand in for it: the
+// client drops a response still on its way when the context ends.
+type late503Transport struct{}
+
+func (late503Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	<-req.Context().Done()
+
+	return &http.Response{
+		StatusCode: http.StatusServiceUnavailable,
+		Body:       http.NoBody,
+	}, nil
+}
+
+// TestGetStopsWhenContextEndsDuringRequest lets the context expire while
+// the first request is still waiting for its 503. get must return the
+// deadline error and keep the 503 with it.
+func TestGetStopsWhenContextEndsDuringRequest(t *testing.T) {
+	t.Parallel()
+
+	client := testClient()
+	client.client.Transport = late503Transport{}
+
+	err := getNothing(client, "http://example.invalid/", 50*time.Millisecond)
+	require.ErrorIs(t, err, context.DeadlineExceeded)
+	require.ErrorIs(t, err, errHTTPStatus)
+}
+
 // TestDownloadFileRetriesToSuccess serves a file that fails twice before
 // it succeeds: first with a 503, then with a body cut off halfway. The
 // download must end with the whole, verified file and no temp file.