[Jtx Board] Adapt to new immutable ical4j.x PropertyList scheme

[rfc2822]

ical4j 4.x changed its collection types (PropertyList, ComponentList, ParameterList) to be immutable — operations like add() and remove() return a new instance instead of modifying in place:

// ical4j 3.x — mutates in place, works
propertyList.add(prop)

// ical4j 4.x — returns new instance, original unchanged
propertyList = propertyList.add(prop)  // must reassign!

Several places in JtxICalObject.kt were discarding those return values, silently losing data (empty VTODO exceptions → HTTP 415 from server, dropped alarms).

This PR

• directly creates the VALARM component with the new properties (alarmProps),
• for exceptions, re-assigns the modified property list.

[Copilot]

Pull request overview

Updates Jtx Board iCalendar generation to work with ical4j 4.x immutable PropertyList behavior, preventing silently dropped exception properties and missing alarm properties when exporting VTODO/VJOURNAL.

Changes:

• Reworks VALARM construction to build a fresh PropertyList from collected alarm properties.
• Fixes recurring exception export by re-assigning the modified propertyList onto the exception component.
• Adds instrumented tests to validate exception UID/RECURRENCE-ID and basic VALARM property presence.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
lib/src/main/kotlin/at/bitfire/ical4android/JtxICalObject.kt	Adapts alarm and recurrence-exception serialization to immutable ical4j list semantics.
lib/src/androidTest/kotlin/at/bitfire/ical4android/JtxICalObjectTest.kt	Adds regression tests for exception properties and alarm property generation.