feat(reward-pool): add approved spender revocation

[chijioke003]

Summary

Adds support for revoking approved RewardPool spenders.

Changes

• Add remove_approved_spender to the RewardPool interface.
• Restrict spender revocation to the stored, authenticated admin.
• Mark revoked spender entries as false in persistent storage.
• Emit a SpenderRemoved event after successful revocation.
• Add coverage confirming:
  • revoked spenders cannot distribute rewards;
  • revoking one spender does not affect other approved spenders;
  • non-admin revocation attempts fail and preserve approval;
  • repeated revocation succeeds and leaves the spender unauthorized.
• Add generated Soroban test snapshots for the new cases.

Testing

cargo test -p reward-pool

Closes #92