[CONTRACT] Add tests for InvalidShipmentInput error variant

[dinahmaccodes]

Tier: Medium

Description: Add tests for invalid shipment input parameters and validate the InvalidShipmentInput error.

Tasks:

• Create shipment with invalid parameter combinations
• Create shipment with valid parameters
• Verify InvalidShipmentInput error (code 17)

Acceptance Criteria:

• Invalid inputs are rejected
• Error variant is returned correctly
• Valid inputs are accepted

Files: contracts/shipment/src/test.rs

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