fix(server): caldav compatibility (#14930)

fix #14411
fix #14909 

Some CalDAV servers do not implement standard responses; add
compatibility for these servers.


#### PR Dependency Tree


* **PR #14930** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Improved CalDAV discovery error handling to gracefully fall back when
the server returns certain error statuses.

* **New Features**
* CalDAV account linking now returns the number of discovered calendars
associated with the account.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/plugins/calendar/service.ts

@@ -272,7 +272,10 @@ export class CalendarService {
       throw error;
     }
 
-    return account;
+    const calendars = await this.models.calendarSubscription.listByAccount(
+      account.id
+    );
+    return { ...account, calendarsCount: calendars.length };
   }
 
   async syncAccountCalendars(accountId: string) {