fix: reject self-transfer in resource_credits::transfer_credits

[Olayinka93]

Overview

This PR adds a protocol-safety guard to resource_credits::transfer_credits so that calls where from == to are rejected. Previously, a self-transfer left balances unchanged but emitted a misleading event and wasted ledger gas. The change rejects self-transfers before any balance mutation or event emission, while preserving authorization, storage/ABI compatibility, and ledger-time behavior.

Related Issue

Changes

🔒 Self-Transfer Guard

• [MODIFY] contracts/resource_credits/src/lib.rs

  • Add a from != to guard immediately after from.require_auth() in transfer_credits.
  • Return Error::SelfTransferNotAllowed when from == to, before any balance reads/writes or event emission.
  • Keep the existing function signature, storage schema, and ABI unchanged.

• [MODIFY] contracts/resource_credits/src/test.rs

  • Add regression test test_transfer_credits_rejects_self_transfer covering the invalid self-transfer path.
  • Extend the success path to verify from != to still transfers normally, updates balances, and emits the expected event.

Verification Results

cargo test -p resource_credits
✅ all tests passed

Live acceptance check:
✅ self-transfer (from == to) rejected
✅ no misleading TransferCredit event emitted
✅ balances unchanged on rejected path
✅ successful transfer between distinct accounts works
✅ authorization/storage/ABI compatibility preserved

Acceptance Criteria	Status
transfer_credits rejects from == to	✅ Guard added before any balance/event side effects
Successful transfer path remains valid	✅ from != to transfers still update balances and emit TransferCredit
No misleading event is emitted	✅ Rejected self-transfer returns error without emitting an event
Authorization is preserved	✅ from.require_auth() remains the first authorization check
Storage/ABI compatibility maintained	✅ No signature or storage layout changes

Closes #253

[Qoder-Voidd]

LGTM