[CONTRACT] Add tests for ProposalExpired error variant

[dinahmaccodes]

Tier: Medium

Description: Add tests for expired proposal operations and validate the ProposalExpired error.

Tasks:

• Attempt to approve expired proposal
• Attempt to execute expired proposal
• Verify ProposalExpired error (code 24)

Acceptance Criteria:

• Expired proposals cannot be approved/executed
• Error variant is returned correctly
• Non-expired proposals work normally

Files: contracts/shipment/src/test.rs

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