[CONTRACT] Add tests for RateLimitExceeded error variant in status updates

[dinahmaccodes]

Tier: Medium

Description: Add tests for status update rate limiting and validate the RateLimitExceeded error.

Tasks:

• Update status multiple times within rate limit window
• Verify RateLimitExceeded error (code 21)
• Wait for window and retry

Acceptance Criteria:

• Rate limit is enforced
• Error variant is returned correctly
• Updates succeed after window expires

Files: contracts/shipment/src/test.rs, contracts/shipment/src/rate_limit.rs

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