fix: CalDAV default calendar not written on booking

[Harshithk951]

What does this PR do?

• Fixes cal.eu / CalDav (Beta) - APP - appointment booking - no appointment in booking calendar cal.eu / CalDav (Beta) - APP - appointment booking - no appointment in booking calendar #28385

When "Add to calendar" is set to "Default" in the CalDAV app, no calendar entry was created on booking even though confirmation emails were sent correctly.

Root cause: the filter in BaseCalendarService.createEvent fell through with : true when no destinationCalendar externalId was set, causing createCalendarObject to fire against every discovered calendar collection. Servers like GMX silently drop these writes so no entry is ever created.

Fix: replace : true with calendars.slice(0, 1) — use only the first (primary) calendar when no explicit destination is configured. The explicit-calendar path is unchanged.

Mandatory Tasks

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A — no API or config changes.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Set up CalDAV app with a GMX or any CalDAV provider
2. Create an event type with "Add to calendar" set to "Default"
3. Book an appointment
4. Verify a calendar entry is created in the provider's calendar

Checklist

• My changes generate no new warnings (Biome + tsc verified)
• PR is under 500 lines / 10 files

[Harshithk951]

@sahitya-chandra I don't have access to a CalDAV provider (e.g. GMX) to record a
reproduction video locally. The fix is a one-line logic change — replacing : true
with calendars.slice(0, 1) in the default calendar filter. Happy to add a unit test
instead if that would be sufficient for review.

[cubic-dev-ai]

No issues found across 1 file

[sahitya-chandra]

Can't approve this without video evidence...

Will you be able to provide it?