Fail fast on invalid config files

tests/config/test_config_load_errors.py

@@ -0,0 +1,30 @@
+import json
+
+import pytest
+
+from nanobot.config.loader import load_config
+
+
+def test_load_config_missing_file_uses_defaults(tmp_path) -> None:
+    config = load_config(tmp_path / "missing.json")
+
+    assert config.agents.defaults.model
+
+
+def test_load_config_invalid_json_fails_fast(tmp_path) -> None:
+    config_path = tmp_path / "config.json"
+    config_path.write_text("{broken json", encoding="utf-8")
+
+    with pytest.raises(ValueError, match="Failed to load config"):
+        load_config(config_path)
+
+
+def test_load_config_invalid_schema_fails_fast(tmp_path) -> None:
+    config_path = tmp_path / "config.json"
+    config_path.write_text(
+        json.dumps({"tools": {"exec": {"timeout": -1}}}),
+        encoding="utf-8",
+    )
+
+    with pytest.raises(ValueError, match="Failed to load config"):
+        load_config(config_path)