Implement Reentrancy Protection for Transfers - #235

Description:

This pr closes #75

This PR enhances the security of the EscrowContract by implementing robust reentrancy protection and strictly adhering to the Checks-Effects-Interactions (CEI) pattern for all functions involving token transfers. This ensures that the platform remains trustless and resilient against recursive call exploits.

Changes:

• Reentrancy Guard: Added a state-based reentrancy guard (Locked flag in instance storage) to prevent nested calls in sensitive functions.
• Checks-Effects-Interactions (CEI): Refactored deposit, release_milestone, release_funds, resolve_dispute, and refund to ensure all internal state updates (e.g., status changes, amount accounting) happen before external token transfers.
• Error Handling: Introduced EscrowError::ReentrancyDetected (12) for graceful handling of unauthorized reentrant calls.
• Comprehensive Logging: Added log! macro calls to all state-changing operations for improved auditability and debugging.
• Documentation: Added a new documentation file docs/contracts/reentrancy_protection.md explaining the implementation details and the security model.

Verification:

• Manually reviewed the logic to ensure the CEI pattern is correctly applied across all transfer paths.
• Confirmed that the reentrancy guard correctly panics and reverts on detection.
• Documentation added to /docs.

Acceptance Criteria:

• Adheres to architectural standards.
• Prevents reentrancy and handles errors gracefully.
• Includes comprehensive logging.
• Minimal on-chain footprint by using instance storage for the guard.