Add Temporal.toInstant() and simplify androidTimeZoneId() to simplify temporal conversion logic

[rfc2822]

This pull request includes several improvements to the temporal conversion logic:

• Adds Temporal.toInstant() method and improves exception handling
• Replaces Instant.ofEpochMilli(toTimestamp()) with toInstant() for cleaner conversions
• Refactors androidTimezoneId() to use a when expression for better readability
• Simplifies recurrence date conversion logic

No test changes required because the logic is only simplified, but not changed.

[Copilot]

Pull request overview

This PR refactors temporal conversion utilities and recurrence-date conversion logic to improve readability and reduce boilerplate when converting between Temporal, Instant, timestamps, and time zones.

Changes:

• Introduces Temporal.toInstant() and refactors other conversions to use it.
• Refactors androidTimezoneId() to a clearer when-style flow and simplifies recurrence date mapping.
• Updates tests to use JUnit’s expected exceptions instead of manual try/catch.

Reviewed changes

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

File	Description
lib/src/main/kotlin/at/bitfire/synctools/util/AndroidTimeUtils.kt	Adds toInstant(), refactors timestamp + timezone conversion logic, and updates exception type.
lib/src/main/kotlin/at/bitfire/synctools/mapping/calendar/builder/AndroidRecurrenceMapper.kt	Simplifies recurrence date conversion via map/when and new conversion helpers.
lib/src/main/kotlin/at/bitfire/synctools/mapping/calendar/builder/DurationBuilder.kt	Switches duration alignment helpers to use toInstant() instead of constructing instants from timestamps.
lib/src/test/kotlin/at/bitfire/synctools/util/AndroidTimeUtilsTest.kt	Updates exception assertions and aligns tests with the new exception types/conversion approach.

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