feat(calendar): list external read-only (subscribed) calendars

list_calendars only kept PROPFIND responses whose resourcetype was
<c:calendar>, so external subscriptions (webcal/ICS feeds) — exposed by
Nextcloud as <cs:subscribed> collections — were silently dropped and never
appeared in nc_calendar_list_calendars (issue #830).

- Construct the calendar DAV client with `X-NC-CalDAV-Webcal-Caching: On` so
  Nextcloud exposes subscriptions as queryable CachedSubscription calendars,
  making their events readable through the existing event/search tools (the
  same mechanism Evolution/KDE desktop clients use).
- In list_calendars, override that header to "Off" so subscriptions are
  returned as cs:subscribed collections with their cs:source href, then parse
  responses whose resourcetype is c:calendar OR cs:subscribed. Subscriptions
  are reported with read_only=True and their source feed URL; their color is
  read from the Apple iCal namespace as a fallback.
- Send the custom PROPFIND markup via `body=` instead of `props=`: under
  caldav 3.x `props=` expects a list of property names and discards a raw XML
  string, producing an empty <prop/>. This also restores display_name,
  description and color which were previously falling back to defaults.
- Add `read_only` and `source` fields to the Calendar model.

Adds unit tests for subscription parsing, the body/header wiring, and the
webcal-caching header; extends the integration test to assert the new fields.

nextcloud_mcp_server/models/calendar.py

@@ -18,6 +18,14 @@ class Calendar(BaseModel):
     timezone: Optional[str] = Field(None, description="Calendar timezone")
     enabled: bool = Field(default=True, description="Whether calendar is enabled")
     ctag: Optional[str] = Field(None, description="Calendar tag for synchronization")
+    read_only: bool = Field(
+        default=False,
+        description="Whether the calendar is read-only (e.g. an external subscription)",
+    )
+    source: Optional[str] = Field(
+        None,
+        description="Source URL of an external/subscribed read-only calendar",
+    )
 
 
 class CalendarEventSummary(BaseModel):