Add rust unit tests for identity, reputation, document contracts

[Johnbliss60]

Closes #1010

Adds comprehensive cargo test coverage for the three Soroban contracts in contracts/ that previously had zero tests, per the issue's acceptance criteria.

• Identity (6 tests): registration, duplicate-wallet rejection, admin revoke, non-admin revoke rejection (via mock_auths), unregistered-wallet lookups
• Reputation (15 tests): rating submission/aggregation, composite score formula, self-rating/duplicate-rating rejection, stats updates, authorization checks
• Document (13 tests): registration, duplicate-hash rejection, integrity verification, admin verify/revoke flows, non-admin rejection

Note: the document contract was missing two pieces the issue's acceptance criteria required — a revoke_document function (with is_revoked field) and duplicate-hash rejection on registration. Both were added as minimal, additive changes alongside their tests.

All tests moved from inline lib.rs modules into dedicated src/test.rs files per the issue's requested convention. cargo test run in contracts/ — all 53 tests pass (34 new + 19 pre-existing in shipment/escrow, unaffected).

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