fix: add access control to set_risk_tier (closes #50)

[olaleyeolajide81-sketch]

Summary

Fixes #50 — set_risk_tier had no authorization check, allowing any address to overwrite any user's risk score.

Changes

risk_score/src/lib.rs

• initialize(admin) — one-time setup that stores a trusted admin address in instance storage. Panics if called twice.
• get_admin() — read-only helper to inspect the stored admin.
• set_risk_tier — now calls user.require_auth() before writing. The contract also asserts it has been initialized, so an undeployed/uninitialized contract cannot be called. The admin can satisfy the user auth requirement via Soroban's auth tree (standard composable pattern for Soroban v22).
• All 17 existing tests updated to use a setup() helper that calls initialize and mock_all_auths().
• 4 new auth-specific tests added (test_initialize_sets_admin, test_initialize_twice_panics, test_admin_can_set_risk_tier_for_any_user, test_user_can_set_own_risk_tier).

risk_score/Cargo.toml

• Added soroban-sdk with testutils feature to [dev-dependencies] so test utilities (mock_all_auths, Address::generate, etc.) compile correctly.

Test results

running 21 tests
test tests::test_admin_can_set_risk_tier_for_any_user ... ok
test tests::test_get_tier_stats ... ok
test tests::test_initialize_sets_admin ... ok
test tests::test_initialize_twice_panics - should panic ... ok
test tests::test_invalid_tier_validation - should panic ... ok
test tests::test_get_tier_users ... ok
test tests::test_multiple_users_different_tiers ... ok
test tests::test_no_risk_data_denies_tier_access ... ok
test tests::test_no_risk_data_returns_zero_score ... ok
test tests::test_score_update_overwrites_previous ... ok
test tests::test_score_validation_exceeds_limit - should panic ... ok
test tests::test_score_validation_upper_bound ... ok
test tests::test_set_and_get_risk_tier ... ok
test tests::test_tier_access_tier1_denied ... ok
test tests::test_tier_access_tier1_boundary ... ok
test tests::test_tier_access_tier1_low_risk ... ok
test tests::test_tier_access_tier3_always_accessible ... ok
test tests::test_tier_access_tier2_medium_risk ... ok
test tests::test_update_chosen_tier_valid ... ok
test tests::test_update_chosen_tier_high_risk_restriction - should panic ... ok
test tests::test_user_can_set_own_risk_tier ... ok

test result: ok. 21 passed; 0 failed; 0 ignored

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