`InvalidPaymentMilestoneName` (#62) is unreachable, and a closed issue's tests lock in the wrong code

[dinahmaccodes]

Tier: Medium
Description: validate_milestone_symbols calls validate_symbol at validation.rs:203 (returns InvalidSymbol/#65 on failure), then calls the same deterministic check again at line 210 mapped to InvalidPaymentMilestoneName/#62 — which can never fire since line 203 already short-circuits. Closed issue #584's resulting tests actually assert Error(Contract, #65), confirming the bug rather than catching it.
Files: contracts/shipment/src/validation.rs:203, 210
Tasks:

• Differentiate the two checks so InvalidPaymentMilestoneName is actually reachable for its intended condition, and correct the [CONTRACT] Add tests for InvalidPaymentMilestoneName error variant #584 tests
  Acceptance Criteria:
• InvalidPaymentMilestoneName is returned for the condition it was designed to catch, verified by a passing test that doesn't merely re-assert the bug