fix: address PR #574 review comments

Restore contact email/birthday/nickname data and per-event calendar
source that were silently dropped during response model wrapping.
Remove dead elif branches in OAuth deck tests, add regression tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextcloud_mcp_server/models/calendar.py

@@ -34,6 +34,12 @@ class CalendarEventSummary(BaseModel):
     status: Optional[str] = Field(
         None, description="Event status (CONFIRMED, TENTATIVE, CANCELLED)"
     )
+    calendar_name: Optional[str] = Field(
+        None, description="Calendar containing this event"
+    )
+    calendar_display_name: Optional[str] = Field(
+        None, description="Display name of calendar containing this event"
+    )
 
 
 class CalendarEvent(CalendarEventSummary):