fix(contract): redact input values in settings errors

Refs #87

tests/test_contracts_settings_privacy.py

@@ -0,0 +1,21 @@
+"""Startup validation errors must not print configuration credentials."""
+
+import pytest
+from pydantic import ValidationError
+
+from app.config import Settings
+
+
+def test_incomplete_max_configuration_does_not_expose_token():
+    with pytest.raises(ValidationError) as error:
+        Settings(_env_file=None, max_enabled=True, max_bot_token="SYNTHETIC_SECRET")
+    message = str(error.value)
+    assert "MAX_BOT_USERNAME" in message
+    assert "SYNTHETIC_SECRET" not in message and "input_value" not in message
+
+
+def test_invalid_field_does_not_expose_accidentally_pasted_secret():
+    with pytest.raises(ValidationError) as error:
+        Settings(_env_file=None, max_enabled="SYNTHETIC_SECRET")
+    assert "max_enabled" in str(error.value)
+    assert "SYNTHETIC_SECRET" not in str(error.value)