[Contract] Automated Contract Emergency Halt Interface

[gabito1451]

Independence Note: Provide the interface and storage flag only. No actual halting of other functions yet.

Context:
In case of an emergency, admins need a single "Big Red Button" to stop all contract interactions globally while they investigate a potential issue.

Objective:
Implement a global 'Halt' flag and interface.

Acceptance Criteria:

• In the state, add an is_halted boolean flag.
• Implement halt_contract(env) and resume_contract(env) secured by require_admin().
• Create a utility ensure_not_halted(env) that panics if the flag is true.
• Write unit tests for setting and clearing the flag.

Technical Pointers:

• This interface is the foundation for the circuit breaker pattern mentioned in other tasks.

[drips-wave]

Congratulations, @Ibinola! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

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[Escelit]

@Escelit has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I can work on this, please assign it to me

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