Fix EventDateTime cast failing on date properties

[chiiya]

Problem

Fixes chiiya/laravel-passes#41.

Adding a dateTime (EventDateTime) to an EventTicketClass and decoding it throws:

EventDateTime::__construct(): Argument #1 ($doorsOpen) must be of type ?string, DateTimeImmutable given

Root cause

In Google\Components\EventTicket\EventDateTime, the doorsOpen, start and end properties carry #[Cast(ISO8601DateCaster::class)], but were typed ?string. DateCaster::unserialize() returns a DateTimeImmutable, which cannot be assigned to a ?string parameter during decode() — hence the TypeError.

This is the inverse of the issue fixed in #51: there the caster returned null into a non-nullable param; here it returns a DateTimeImmutable into a string param.

Fix

Type the three date properties DateTimeInterface|string|null, matching Common\DateTime::$date. The cast result now fits, and DateCaster::serialize() already handles both DateTimeInterface and string inputs, so building/encoding is unaffected (round-trip still emits canonical ISO8601 strings).

Tests

Added tests/Google/Components/EventTicket/EventDateTimeTest.php:

• decoding ISO8601 strings yields DateTimeImmutable values
• round-trip decode → serialize emits canonical ISO8601 strings
• constructing with plain date strings still works

Full suite passes (54 tests); just lint clean.