fix: pool pause() is not idempotent — can be called multiple times with no error

[thewealthyplace]

Bug

contracts/pool/src/lib.rs — pause() sets a Paused storage flag to true but does not check whether the contract is already paused. Calling pause() on an already-paused contract:

1. Succeeds silently (no error, no event distinguishing a double-pause from first pause)
2. Wastes gas on a no-op write
3. Could confuse monitoring systems that count pause events

Same issue applies to unpause() on an already-unpaused contract.

Fix

pub fn pause(env: &Env, admin: Address) {
    admin.require_auth();
    verify_admin(&env, &admin);
    
    if is_paused(&env) {
        return Err(PoolError::AlreadyPaused);
    }
    
    env.storage().persistent().set(&DataKey::Paused, &true);
    env.events().publish((Symbol::new(&env, "POOL"), Symbol::new(&env, "paused")), admin);
}

pub fn unpause(env: &Env, admin: Address) {
    admin.require_auth();
    verify_admin(&env, &admin);
    
    if !is_paused(&env) {
        return Err(PoolError::NotPaused);
    }
    
    env.storage().persistent().set(&DataKey::Paused, &false);
    env.events().publish((Symbol::new(&env, "POOL"), Symbol::new(&env, "unpaused")), admin);
}

Apply the same fix to the invoice and credit score contracts.

Acceptance Criteria

• pause() returns AlreadyPaused when already paused
• unpause() returns NotPaused when already unpaused
• Applied to all three contracts
• Unit tests for idempotency cases

References

• contracts/pool/src/lib.rs, contracts/invoice/src/lib.rs, contracts/credit_score/src/lib.rs — pause(), unpause()