Optimize the kani formal verification steps

[ChristopherRabotin]

As per #289 , it takes over two hours to run the Kani model checker. This task is to optimize this test so we can run it in reasonable time.

[ChristopherRabotin]

Turns out that two of the Kani tests fail: https://github.com/nyx-space/hifitime/actions/runs/8827134540/job/24233914219?pr=298

This ticket needs to address that as well.

[ChristopherRabotin]

This ticket will now need to focus on fixing the bugs found in Kani; the CI for Kani now runs once a week.

[ChristopherRabotin]

Test kani_harness_is_gregorian_valid fails : https://github.com/nyx-space/hifitime/actions/runs/15800867110/job/44538839116