test: add tests for YAML config loading

- test_config.py: covers load_yaml_config (happy path, missing file raises)
- Uses project-relative paths so tests run in any environment

tests/test_config.py

@@ -0,0 +1,38 @@
+"""Tests for YAML config loading."""
+
+import sys
+from pathlib import Path
+
+import pytest
+
+# Ensure src is on the path when running from the project root
+sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
+
+CONFIGS_DIR = Path(__file__).parent.parent / "configs"
+
+
+class TestYamlConfig:
+    def test_load_yaml_config(self):
+        from experiment_logger.config_utils import load_yaml_config
+
+        config = load_yaml_config(str(CONFIGS_DIR / "default_ppo.yaml"))
+        assert isinstance(config, dict)
+        assert "total_timesteps" in config
+        assert "learning_rate" in config
+
+    def test_load_dev_test_config(self):
+        from experiment_logger.config_utils import load_yaml_config
+
+        config = load_yaml_config(str(CONFIGS_DIR / "dev_test.yaml"))
+        assert config["total_timesteps"] == 100000
+
+    def test_missing_config_raises(self):
+        from experiment_logger.config_utils import load_yaml_config
+
+        with pytest.raises(FileNotFoundError):
+            load_yaml_config("nonexistent.yaml")
+
+    def test_merge_config_with_cli_is_callable(self):
+        from experiment_logger.config_utils import merge_config_with_cli
+
+        assert callable(merge_config_with_cli)