feat(core): add generic StateMachine base class for typed state transitions

[ryuwd]

Reusable declarative state machine utility for diracx. Each system subclasses StateMachine with StrEnum states and a transitions dict. Validation runs at class creation via init_subclass.

[aldbr]

Is there any github issue linked to your PR?
Can you briefly explain why you need that, and why now please?

[aldbr]

Given that:

• this is not an urgent or straightforward fix,
• it is not part of the current sprint planning,
• it is not backed by a documented user story (I understand the intention, but the use case is not documented here for people outside LHCb),
• it may require a broader discussion about the approach (for instance, there are existing Python libraries dealing with state machines, such as https://github.com/fgmacedo/python-statemachine),

I'm closing this PR for now.

Please feel free to reopen it once a proper GitHub issue describing the use case has been created and the topic has been discussed in a ddev meeting.

Thank you for your understanding.

[ryuwd]

I understand the reasons for closing, particularly since more discussion and design is needed, but it's clear that a more modern solution and cleanup will eventually be needed for DiracX as from my experience moving the job routes (including job status transitions) we shoot ourselves in the foot by continuing to rely on the legacy impl and behaviours.