Events: add VALUE=DATE to EXDATE for cancelled all-day exceptions

[rfc2822]

No description provided.

[Copilot]

Pull request overview

Ensures that cancelled all-day recurring exceptions are exported as EXDATE entries with the correct iCalendar value type (VALUE=DATE), matching RFC 5545 expectations and improving interoperability with clients that require explicit VALUE=DATE for date-only exclusions.

Changes:

• Update AndroidEventHandler.asExDate() to emit EXDATE;VALUE=DATE:<yyyymmdd> for cancelled all-day exceptions.
• Strengthen recurrence-related tests to assert presence of VALUE=DATE on all-day EXDATE.
• Add a dedicated mapping test case for cancelled all-day exceptions to prevent regression.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
lib/src/main/kotlin/at/bitfire/synctools/mapping/calendar/AndroidEventHandler.kt	Adds VALUE=DATE parameter when generating EXDATE for cancelled all-day exceptions.
lib/src/test/kotlin/at/bitfire/synctools/mapping/calendar/AndroidEventHandlerTest.kt	Adds test coverage asserting EXDATE uses VALUE=DATE for cancelled all-day exceptions.
lib/src/test/kotlin/at/bitfire/synctools/mapping/calendar/handler/RecurrenceFieldHandlerTest.kt	Updates all-day recurrence test to assert EXDATE includes VALUE=DATE.