CompletedBuilder: Fix LocalDateTime cannot be cast to Instant

[sunkup]

Fix ClassCastException in CompletedBuilder : DmfsTaskFieldBuilder when DmfsTaskBuilder.buildTask runs.

The exception users get is ClassCastException: LocalDateTime cannot be cast to Instant so must occur where code assumes Instant at runtime but gets LocalDateTime. DmfsTaskBuilder.buildTask loops over all fieldBuilders calling .build()` so the crash must be inside one of them.

StartTimeBuilder and DueBuilder use normalizedDate()?.toTimestamp(), but CompletedBuilder does not. It uses .date?.toEpochMilli() which works only on Instant (!), so any non-Instant temporal here causes exactly this exception.

Relevant changes made:

• call completedAt?.normalizedDate()?.toTimestamp() in COMPLETED builder instead of completedAt?.date?.toEpochMilli().
• Add a regression test which demonstrates the bug and ensures it does not come back

Also:

• AndroidTimeUtils: Don't lose milliseconds in toTimestamp which was loosing milliseconds. Changed toInstant().epochSecond * 1000 to toInstant().toEpochMilli().

[Copilot]

Pull request overview

Fixes a runtime ClassCastException in the DMFS task mapping when building the COMPLETED field from iCalendar data by normalizing temporals before converting them to provider timestamps.

Changes:

• Update CompletedBuilder to convert completedAt via normalizedDate() + toTimestamp() (instead of assuming Instant).
• Add a regression test covering floating COMPLETED (timezone-less) date-times to ensure the builder does not crash and produces the expected timestamp.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/src/main/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilder.kt	Fixes COMPLETED timestamp generation by normalizing temporals and using the shared timestamp conversion helper.
lib/src/test/kotlin/at/bitfire/synctools/mapping/tasks/builder/CompletedBuilderTest.kt	Adds a regression test for floating COMPLETED values to prevent reintroducing the crash.