[CONTRACT] Add tests for MilestoneSumInvalid error variant

[dinahmaccodes]

Tier: Medium

Description: Add tests for milestone percentage sum validation and validate the MilestoneSumInvalid error.

Tasks:

• Create shipment with milestones summing to != 100%
• Create shipment with milestones summing to 100%
• Verify MilestoneSumInvalid error (code 18)

Acceptance Criteria:

• Invalid milestone sums are rejected
• Error variant is returned correctly
• Valid milestone sums are accepted

Files: contracts/shipment/src/test.rs

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