[CONTRACT] Add tests for ProposalAlreadyExecuted error variant

[dinahmaccodes]

Tier: Medium

Description: Add tests for duplicate proposal execution and validate the ProposalAlreadyExecuted error.

Tasks:

• Execute proposal twice
• Verify ProposalAlreadyExecuted error (code 23)
• Test single execution succeeds

Acceptance Criteria:

• Duplicate executions are rejected
• Error variant is returned correctly
• Single execution succeeds

Files: contracts/shipment/src/test.rs

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