[CONTRACT] Add tests for ReentrancyDetected error variant

[dinahmaccodes]

Tier: Hard

Description: Add tests for reentrancy detection and validate the ReentrancyDetected error.

Tasks:

• Attempt reentrant calls to escrow operations
• Verify ReentrancyDetected error (code 15)
• Test normal non-reentrant calls

Acceptance Criteria:

• Reentrant calls are detected and rejected
• Error variant is returned correctly
• Normal calls succeed

Files: contracts/shipment/src/test.rs, contracts/shipment/src/test_reentrancy_guard.rs

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