fix: prevent self-transfer from stripping Admin in transfer_authority

[Joeljosef7]

Overview

This PR fixes a privilege-loss bug in DripGovernor::transfer_authority (contracts/governor/src/lib.rs:302-313). When a caller transfers authority to themselves, the existing grant short-circuits as a no-op and the subsequent revoke removes the caller's own Admin role, decrementing AdminCount. The fix rejects new_authority == caller and is_zero_stellar_account(new_authority) with Error::InvalidParam before any role mutation, preserving AdminCount and the caller's Admin role on invalid input.

Related Issue

Changes

🛡️ Authority Transfer Guards

• [MODIFY] contracts/governor/src/lib.rs
  • Add early guard in transfer_authority rejecting new_authority == caller with Error::InvalidParam before grant/revoke are called.
  • Add zero-address rejection via is_zero_stellar_account(new_authority) with Error::InvalidParam, consistent with the propose_authority guard from propose_authority accepts the zero address and silently overwrites a pending proposal #338.
  • Keep the deprecated one-step path functionally intact for legitimate transfers; no role state is mutated on invalid input.

Verification Results

cargo test --package drip-governor
✅ 24/24 passed

Live acceptance check:
✅ Self-transfer (new_authority == caller) rejected with Error::InvalidParam
✅ Zero-address transfer rejected with Error::InvalidParam
✅ Legitimate transfer still grants new Admin and revokes caller
✅ AdminCount unchanged on rejected paths

Acceptance Criteria	Status
transfer_authority rejects new_authority == caller with Error::InvalidParam before mutating roles	✅ Guard added before any role mutation
transfer_authority rejects zero-address new_authority with Error::InvalidParam	✅ is_zero_stellar_account check added
Caller's Admin role and AdminCount are never modified on invalid input	✅ grant/revoke only execute after validation passes
Existing valid authority transfers continue to work	✅ Legacy one-step path preserved unchanged for valid inputs

Closes #359

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