fix(security): mitigate reentrancy in transfer operations

[mofejo1]

Summary

• add a shared reentrancy guard utility and module-level lock keys
• enforce checks-effects-interactions in transfer-heavy paths across modules
• protect transfer flows in:
  • contracts/teachlink/src/rewards.rs
  • contracts/teachlink/src/bridge.rs
  • contracts/teachlink/src/atomic_swap.rs
  • contracts/teachlink/src/insurance.rs
• add focused unit tests that assert guard-trigger behavior for critical transfer entry points

Security changes

• introduced reentrancy::with_guard(...) wrapper to prevent nested execution on protected code paths
• moved state mutation ahead of external token calls in vulnerable flows (CEI)
• added explicit reentrancy error variants:
  • BridgeError::ReentrancyDetected
  • EscrowError::ReentrancyDetected
  • RewardsError::ReentrancyDetected

Validation

• cargo check -p teachlink-contract --lib ✅
• full cargo test could not be completed locally due disk exhaustion from this repository’s heavy build graph in current environment

Closes #229

[drips-wave]

@mofejo1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits