notify: a reply other than 2xx is a failed delivery (closes #287)

A notification endpoint that answered with a redirect was counted as
delivered: the request is not sent through a client, so the redirect
is never followed, and only a status of 400 or more counted as a
failure. Now any status outside 200-299 fails the delivery for ntfy,
Slack and Mattermost, so it is retried and logged like the others.

Model: opus-5-5

internal/notify/delivery_test.go

@@ -1185,6 +1185,42 @@ func sendToEachEndpoint(t *testing.T, baseURL, title string) string {
 	return logs.String()
 }
 
+// assertEachEndpointRetriedThenFailed fails the test unless, for each
+// of ntfy, Slack and Mattermost, the lines of logs whose error
+// contains reason are the retry line and then the final failure line.
+func assertEachEndpointRetriedThenFailed(
+	t *testing.T, logs, reason string,
+) {
+	t.Helper()
+
+	// For each endpoint, the messages of its lines whose error
+	// contains reason, in the order they were logged.
+	got := map[string][]string{}
+
+	for _, record := range parseLog(t, logs) {
+		if strings.Contains(record.Error, reason) {
+			got[record.Endpoint] = append(
+				got[record.Endpoint], record.Msg,
+			)
+		}
+	}
+
+	lines := []string{
+		"notification delivery failed, retrying",
+		"failed to send notification after retries",
+	}
+	want := map[string][]string{
+		endpointNtfy:       lines,
+		endpointSlack:      lines,
+		endpointMattermost: lines,
+	}
+
+	if !maps.EqualFunc(got, want, slices.Equal) {
+		t.Errorf("lines whose error contains %q = %v, want %v",
+			reason, got, want)
+	}
+}
+
 // TestSendNotificationDebugLogOmitsWebhookURL checks that, with debug
 // logging on, a delivery to each endpoint logs the endpoint's name and
 // the title, and no part of the URL's path or query.
@@ -1240,33 +1276,7 @@ func TestSendNotificationErrorLogOmitsWebhookURL(t *testing.T) {
 	logs := sendToEachEndpoint(t, srv.URL, "t")
 
 	assertNoWebhookSecret(t, logs)
-
-	// For each endpoint, the messages of its lines whose error names
-	// the refused connection, in the order they were logged.
-	got := map[string][]string{}
-
-	for _, record := range parseLog(t, logs) {
-		if strings.Contains(record.Error, "connection refused") {
-			got[record.Endpoint] = append(
-				got[record.Endpoint], record.Msg,
-			)
-		}
-	}
-
-	lines := []string{
-		"notification delivery failed, retrying",
-		"failed to send notification after retries",
-	}
-	want := map[string][]string{
-		endpointNtfy:       lines,
-		endpointSlack:      lines,
-		endpointMattermost: lines,
-	}
-
-	if !maps.EqualFunc(got, want, slices.Equal) {
-		t.Errorf("lines whose error names the refused connection = %v, "+
-			"want %v", got, want)
-	}
+	assertEachEndpointRetriedThenFailed(t, logs, "connection refused")
 }
 
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