Add support for Julia v1.12 by fixing JET type stability warning

[albertomercurio]

Description

This PR adds support for Julia v1.12 by fixing a JET static analysis warning that was introduced with stricter world age semantics in Julia 1.12.

Changes

• Added ::Diagonal type annotation to Id_cache parameter in liouvillian function in src/qobj/superoperators.jl

Problem

Julia 1.12 introduced more strict world age semantics for global bindings, causing JET to report:

WARNING: Detected access to binding `QuantumToolbox.liouvillian` in a world prior to its definition world.
WARNING: Detected access to binding `QuantumToolbox._sum_lindblad_dissipators` in a world prior to its definition world.
no matching method found `_sum_lindblad_dissipators(::Tuple, ::Nothing)`, 
`_sum_lindblad_dissipators(::AbstractVector, ::Nothing)`, etc.

Solution

By adding the ::Diagonal type annotation to Id_cache, Julia can now properly infer the type at compile time, eliminating the type instability that was causing issues with Julia 1.12's stricter semantics.

Testing

All code quality tests now pass:

• ✅ Aqua.jl: 9/9 tests passed
• ✅ JET.jl: 1/1 test passed