Deregister the guardian from AllGuardians and the slot index in resign_guardian — a resigned guardian can never be re-added

[N-thnI]

Description

Make resign_guardian clean up guardian membership the same way remove_guardian does.

Problem Statement

resign_guardian clears only the membership flag (src/contracts/vault_ops.rs:134-135):

let g_key = DataKey::Guardian(guardian.clone());
env.storage().instance().remove(&g_key);

It never removes the address from DataKey::AllGuardians, nor from the GuardianIndexAt/GuardianIndexOf/GuardianIndexCount slot index — all of which guardian::remove_guardian (src/guardian.rs:62-133) cleans up properly.

Verified against current main: after a guardian resigns, is_guardian is false, but get_snapshot_meta().guardian_count is still 1, get_snapshot().guardians still holds 1 entry, and add_guardian(&admin, &g) returns Err(DuplicateGuardian) — because add_guardian checks all_guardians.contains(guardian) at src/guardian.rs:25, which still holds.

So a guardian who resigns is permanently locked out of the protocol, and the leaked slot inflates guardian_count and get_guardians_page indefinitely.

Proposed Changes

• Have resign_guardian call guardian::remove_guardian (or a shared deregister_guardian helper) instead of the bare remove(&g_key), so AllGuardians and the slot index are compacted identically
• Keep the token-refund/timelock logic after deregistration, and preserve the existing NotGuardian early return
• Add a regression test asserting a resigned address can be re-added

Technical Implementation Scaffolding

• Target Repository: vero-core-contracts
• Target Path: src/contracts/vault_ops.rs, src/guardian.rs
• Branch Naming: fix/issue--resign-guardian-deregister
• Authority Context: Correctness — guardian membership; locks users out permanently

Acceptance Criteria

• After resign_guardian, get_snapshot_meta().guardian_count decreases by 1 and the address no longer appears in get_guardians_page(0, 50)
• add_guardian(admin, g) succeeds after g resigned, and is_guardian(g) is then true
• resign_guardian and remove_guardian leave byte-identical guardian-index state for the same address

Definition of Done

• Reviewed by lead maintainer
• Pull request merged via verified status check

[Joycejay17]

Hey there maintainer, I am a software engineer with a lot of blockchain experience. I have read through the Issue description and with my experience in stellar projects can get this delivered ASAP.

[grantfox-oss]

🦊 GrantFox — @Joycejay17 has been assigned to this issue as part of the Third Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #303)
2. Your PR will be reviewed by the Vero-protocol maintainers

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