Stop one recurring event from taking its whole calendar down

An RRULE was parsed by splitting on ";" and "=", which leaves every value a
str. icalendar accepts that at add() time and only rejects it inside
to_ical(), where vRecur.types maps UNTIL to vDDDTypes and that demands a date.
So a single "repeats until <date>" event — exactly what the web editor writes —
made build_ics raise ValueError, and with it every REPORT, every event GET,
the whole-collection GET and the .ics export for that calendar.

PROPFIND never builds ICS, so discovery kept working. That is the asymmetry
behind the Home Assistant symptom: the calendar is found under its correct
name, then every update fails and the entity sits permanently unavailable. It
behaved identically through the token URL and the login URL because both reach
the same collection dispatch.

icalendar parses its own format correctly, so the rule now goes through
vRecur.from_ical. That parser can raise where the hand-rolled one silently
accepted anything, so an unreadable stored rule now drops the recurrence and
logs it instead of failing the calendar — the same tolerance the DTSTART and
DTEND parsing already has.

caldav_client carried a second copy of the same parser with the same defect,
used when pushing events to external CalDAV servers. Deleted; both directions
now share one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/caldav_client.py

@@ -4,7 +4,11 @@ from datetime import date, datetime, timedelta, timezone
 from typing import Dict, List, Optional
 
 import caldav
-from icalendar import Calendar, Event, vDatetime, vRecur
+from icalendar import Calendar, Event, vDatetime
+
+# One parser for both directions: this file used to carry its own copy with
+# the same UNTIL defect.
+from ical_io import _rrule_str_to_vrecur as _parse_rrule_str
 
 logger = logging.getLogger(__name__)
 
@@ -306,20 +310,6 @@ def delete_event(url: str, username: str, password: str, event_url: str,
     resource.delete()
 
 
-def _parse_rrule_str(rrule_str: str) -> vRecur:
-    """Parse an RRULE string like 'FREQ=WEEKLY;BYDAY=MO,WE' into a vRecur."""
-    params = {}
-    for part in rrule_str.split(";"):
-        if "=" not in part:
-            continue
-        key, val = part.split("=", 1)
-        if "," in val:
-            params[key] = val.split(",")
-        else:
-            params[key] = val
-    return vRecur(params)
-
-
 def _parse_dt(s: str) -> datetime:
     s = s.replace("Z", "+00:00")
     dt = datetime.fromisoformat(s)