[RewardPool] Revoke approved spender access

[Kaylahray]

Description

The RewardPool can add approved spender contracts, but it also needs an admin-controlled revoke path so compromised or deprecated spenders can be removed immediately.

User Story

As a Protocol Admin,
I want to revoke a spender's authorization,
so that outdated or compromised contracts can no longer distribute rewards.

Requirements and Context

• Files: contracts/reward-pool/src/lib.rs, contracts/reward-pool/src/types.rs, contracts/reward-pool/src/test.rs
• Add remove_approved_spender(env: Env, admin: Address, spender: Address)
• Require admin auth and stored-admin verification
• Ensure distribute_reward fails immediately for revoked spenders

Suggested Implementation

// IN FUNCTION: remove_approved_spender(env: Env, admin: Address, spender: Address)
// 1. admin.require_auth();
// 2. verify admin matches stored admin;
// 3. env.storage().persistent().set(&DataKey::Spender(spender), &false);
// 4. emit spender-revoked event.

Acceptance Criteria

• Revoked spenders can no longer call distribute_reward successfully.
• Other approved spenders remain unaffected.
• Revocation is idempotent or otherwise documented clearly.

Submission Guidelines

• Branch: feat/revoke-approved-spender
• Depends on: RewardPool add_approved_spender function
• PR: feat(reward-pool): add approved spender revocation

[Martins-594]

Hi, I'd like to work on this issue. I've gone through the description and acceptance criteria and I'm confident I can deliver it. I'll make sure every acceptance criterion is met and submit a clean, tested PR. Please assign it to me.

[malaysiaonelove]

Hi maintain,I will love to work on this issue,I have an experience and I have already reviewed the issue please assigned me to it and I will give a good result.

[chijioke003]

Hi maintainer, can you assign this issue to me

[grantfox-oss]

🦊 GrantFox — @chijioke003 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #92)
2. Your PR will be reviewed by the learnault maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@chijioke003's PR #105 was approved and merged by @Kaylahray.

🏆 @chijioke003: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (45 total points). Track your full progress on GrantFox.

👏 Great work, @chijioke003! Keep contributing to learnault.