[CONTRACT] Add tests for ProposalNotFound error variant

[dinahmaccodes]

Tier: Medium

Description: Add tests for operations on non-existent proposals and validate the ProposalNotFound error.

Tasks:

• Attempt to approve non-existent proposal
• Attempt to execute non-existent proposal
• Verify ProposalNotFound error (code 22)

Acceptance Criteria:

• Non-existent proposals return correct error
• Error variant is returned correctly
• Existing proposals work normally

Files: contracts/shipment/src/test.rs

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