test(#419): add unit tests for key transfer sender balance decremented correctly

creator-keys/tests/transfer_sender_balance.rs

@@ -0,0 +1,55 @@
+//! Unit tests: transfer must decrement sender balance by exact transferred amount.
+
+mod contract_test_env;
+use contract_test_env::{
+    register_creator_keys, register_test_creator, test_env_with_auths,
+    set_key_price_for_tests, set_protocol_fee_bps, compute_expected_buy_price,
+};
+use soroban_sdk::{testutils::Address as _, Address};
+
+#[test]
+fn transfer_decrements_sender_balance_by_exact_amount() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    set_key_price_for_tests(&env, &client, 100i128);
+    set_protocol_fee_bps(&env, &client, 9000u32, 1000u32);
+    let creator = register_test_creator(&env, &client);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    // Buy 3 keys
+    for i in 0..3i64 {
+        let price = compute_expected_buy_price(i, 100i128);
+        client.buy_key(&creator, &sender, &price, &None);
+    }
+    let balance_before = client.get_key_balance(&creator, &sender);
+    assert_eq!(balance_before, 3);
+
+    // Transfer 2 keys
+    client.transfer_key(&creator, &sender, &recipient, &2i128);
+
+    let balance_after = client.get_key_balance(&creator, &sender);
+    assert_eq!(balance_after, 1,
+        "Sender balance must be decremented by exactly 2");
+    assert_eq!(client.get_key_balance(&creator, &recipient), 2);
+}
+
+#[test]
+fn transfer_full_balance_leaves_sender_at_zero() {
+    let env = test_env_with_auths();
+    let (client, _) = register_creator_keys(&env);
+    set_key_price_for_tests(&env, &client, 100i128);
+    set_protocol_fee_bps(&env, &client, 9000u32, 1000u32);
+    let creator = register_test_creator(&env, &client);
+    let sender = Address::generate(&env);
+    let recipient = Address::generate(&env);
+
+    let price = compute_expected_buy_price(0, 100i128);
+    client.buy_key(&creator, &sender, &price, &None);
+    assert_eq!(client.get_key_balance(&creator, &sender), 1);
+
+    client.transfer_key(&creator, &sender, &recipient, &1i128);
+
+    assert_eq!(client.get_key_balance(&creator, &sender), 0,
+        "Sender balance must be zero after full transfer");
+}