Fix loan account conversion invariant

tests/test_account_valuation_edge_contracts.py

@@ -5,6 +5,39 @@ from jarca_ledger_mcp.application.account_valuation_service import record_accoun
 from jarca_ledger_mcp.application.imports_service import _record_import_valuation_candidate
 
 
+def test_bank_to_loan_conversion_rejects_existing_undefined_valuation_without_mutation(tmp_path):
+    path = str(tmp_path / "ledger.sqlite3")
+    created = service.create_ledger_account(
+        "은행", type="bank", last_balance=100, last_synced_at="2026-06-30",
+        last_balance_basis_date="2026-06-30", db_path=path,
+    )
+    account_id = created["account"]["id"]
+    before = service.list_account_valuations(account_id, db_path=path)
+    assert before["count"] == 1
+    assert before["valuations"][0]["representative_liability_definition"] is None
+
+    rejected = service.update_ledger_account(account_id, type="loan", db_path=path)
+
+    assert rejected["success"] is False
+    assert rejected["error_code"] == "validation_error"
+    accounts = service.list_ledger_accounts(active_only=False, db_path=path)["accounts"]
+    assert next(account for account in accounts if account["id"] == account_id)["type"] == "bank"
+    after = service.list_account_valuations(account_id, db_path=path)
+    assert after["count"] == 1
+    assert after["valuations"] == before["valuations"]
+
+
+def test_bank_to_loan_conversion_without_valuations_remains_allowed(tmp_path):
+    path = str(tmp_path / "ledger.sqlite3")
+    account_id = service.create_ledger_account("은행", type="bank", db_path=path)["account"]["id"]
+
+    converted = service.update_ledger_account(account_id, type="loan", db_path=path)
+
+    assert converted["success"] is True
+    assert converted["account"]["type"] == "loan"
+    assert service.list_account_valuations(account_id, db_path=path)["count"] == 0
+
+
 def test_loan_balance_requires_representative_liability_definition(tmp_path):
     path = str(tmp_path / "ledger.sqlite3")
     bank = service.create_ledger_account("은행", type="bank", db_path=path)["account"]["id"]