Harden DAV outbound URL validation (#2819)

tests/test_caldav_url_nonstring.py

@@ -5,9 +5,13 @@ It did `(raw_url or "").strip()`, so a non-string scalar (e.g. an int from a
 mis-typed config) reached `.strip()` and raised TypeError instead of the
 function\'s own ValueError.
 """
+import ipaddress
+
 import pytest
 
-from src.caldav_sync import validate_caldav_url
+from src import caldav_sync
+
+validate_caldav_url = caldav_sync.validate_caldav_url
 
 
 def test_non_string_raises_valueerror_not_typeerror():
@@ -17,6 +21,11 @@ def test_non_string_raises_valueerror_not_typeerror():
         validate_caldav_url(None)
 
 
-def test_valid_url_passes():
+def test_valid_url_passes(monkeypatch):
+    monkeypatch.setattr(
+        caldav_sync,
+        "_resolve_caldav_host_ips",
+        lambda host: [ipaddress.ip_address("93.184.216.34")],
+    )
     out = validate_caldav_url("https://dav.example.com/calendars/")
     assert "example.com" in out