Add tests for ProductState structure

[rizumita]

Description

The ProductState struct is used in product automaton construction but has no dedicated tests. This is a fundamental structure for model checking algorithms.

Tasks

• Create ProductStateRepresentationTests.swift in the ModelChecking tests directory
• Test creation with various state type combinations
• Test equality for different state types
• Test hash consistency
• Test with custom Hashable types
• Verify proper generic type handling

Priority

Medium

Location

Sources/TemporalKit/ModelChecking/ProductStateRepresentation.swift