feat: add config file loading with error handling

Implement load_config() function with comprehensive error handling
- Loads and parses JSON config files
- Raises ConfigValidationError for missing files
- Raises ConfigValidationError for malformed JSON
- Includes 3 passing tests for all error cases

Test coverage:
- test_load_config_valid_json: Verifies successful JSON parsing
- test_load_config_file_not_found: Validates error on missing file
- test_load_config_invalid_json: Validates error on malformed JSON

tests/unit/test_config_merger.py

@@ -0,0 +1,37 @@
+import pytest
+import json
+import tempfile
+from pathlib import Path
+from tools.config_merger import load_config, ConfigValidationError
+
+
+def test_load_config_valid_json():
+    """Test loading a valid JSON config file"""
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+        json.dump({"key": "value"}, f)
+        temp_path = f.name
+
+    try:
+        result = load_config(temp_path)
+        assert result == {"key": "value"}
+    finally:
+        Path(temp_path).unlink()
+
+
+def test_load_config_file_not_found():
+    """Test loading non-existent config file"""
+    with pytest.raises(ConfigValidationError, match="not found"):
+        load_config("/nonexistent/path.json")
+
+
+def test_load_config_invalid_json():
+    """Test loading malformed JSON"""
+    with tempfile.NamedTemporaryFile(mode='w', suffix='.json', delete=False) as f:
+        f.write("{invalid json")
+        temp_path = f.name
+
+    try:
+        with pytest.raises(ConfigValidationError, match="Invalid JSON"):
+            load_config(temp_path)
+    finally:
+        Path(temp_path).unlink()