[12.x] Add PHPDoc callable types for BusFake methods

[alies-dev]

Add proper callable type hints that document the callback signatures for batch-related methods in BusFake and ChainedBatchTruthTest.

Changes:

• BusFake::chainedBatch() - @param \Closure(\Illuminate\Bus\PendingBatch): bool
• BusFake::assertBatched() - @param callable(\Illuminate\Bus\PendingBatch): bool
• BusFake::batched() - @param callable(\Illuminate\Bus\PendingBatch): bool and @return \Illuminate\Support\Collection<int, \Illuminate\Bus\PendingBatch>
• ChainedBatchTruthTest::$callback and constructor - @var/@param \Closure(\Illuminate\Bus\PendingBatch): bool

This improves IDE autocompletion and static analysis support when using these testing methods.