Suppress GWR busy-page error emails and add regression tests

tests/test_update_gwr.py

@@ -0,0 +1,57 @@
+"""Regression tests for GWR update notifications."""
+
+from pathlib import Path
+from unittest.mock import Mock, patch
+
+import flask
+import pytest
+
+from update import update_gwr_advance_ticket_date
+
+
+@pytest.mark.parametrize(
+    ("response_html", "expect_email"),
+    [
+        (
+            "<p>We are sorry that we cannot show you the page you were looking for. "
+            "Our website is extremely busy at the moment.</p>",
+            False,
+        ),
+        (
+            "<p>We are sorry that we cannot show you the page you were looking for.\n"
+            "Our website is   extremely busy at the moment.</p>",
+            False,
+        ),
+        ("<p>Unexpected response without booking dates.</p>", True),
+    ],
+    ids=["busy-page", "busy-page-whitespace", "unexpected-page"],
+)
+def test_gwr_missing_dates_notifications(
+    tmp_path: Path, response_html: str, expect_email: bool
+) -> None:
+    """Only known busy pages skip alerts; failed responses preserve the cache."""
+    cached_html = """
+    <table>
+      <tr><td>Weekdays</td><td>Friday 25 December 2026</td></tr>
+      <tr><td>Saturdays</td><td>Saturday 26 December 2026</td></tr>
+      <tr><td>Sundays</td><td>Sunday 27 December 2026</td></tr>
+    </table>
+    """
+    cache = tmp_path / "advance-tickets.html"
+    cache.write_text(cached_html)
+    config = flask.config.Config(str(tmp_path))
+    config["DATA_DIR"] = str(tmp_path)
+
+    with (
+        patch("update.requests.get", return_value=Mock(text=response_html)),
+        patch("update.agenda.mail.send_mail") as send_mail,
+    ):
+        update_gwr_advance_ticket_date(config)
+
+    if expect_email:
+        send_mail.assert_called_once_with(
+            config, "Error parsing GWR advance ticket booking dates", response_html
+        )
+    else:
+        send_mail.assert_not_called()
+    assert cache.read_text() == cached_html