fix(contract): redact input values in settings errors

Refs #87

app/config.py

@@ -14,7 +14,7 @@ ROOT_DIR = Path(__file__).resolve().parent.parent
 
 class Settings(BaseSettings):
     # Empty values in .env (e.g. `TG_BOT_TOKEN=`) fall back to defaults instead of failing validation.
-    model_config = SettingsConfigDict(env_file=".env", extra="ignore", env_ignore_empty=True)
+    model_config = SettingsConfigDict(env_file=".env", extra="ignore", env_ignore_empty=True, hide_input_in_errors=True)
 
     # Two separate databases (ADR-003): identity holds direct identifiers, clinical holds pseudonymised data.
     identity_db_url: str = "postgresql+asyncpg://identity_app:identity_app@localhost:5432/identity"

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)