chore(pr): address code review feedback for PR #420

Added CHANGELOG entry for CLI extension test suite. Added Brent Edwards
to CONTRIBUTORS.md. Replaced broad except Exception with specific
ValidationError catch in benchmark. Moved import json to module
top-level in robot helper script.

ISSUES CLOSED: #326

robot/helper_cli_extensions.py

@@ -5,6 +5,7 @@ Each subcommand is a self-contained check that prints a sentinel on success.
 
 from __future__ import annotations
 
+import json
 import sys
 from datetime import datetime
 from pathlib import Path
@@ -288,8 +289,6 @@ def action_show_extended() -> None:
 
 def action_show_json() -> None:
     """Verify action show JSON output includes optional fields."""
-    import json
-
     mock_svc = MagicMock()
     mock_svc.get_action_by_name.return_value = _mock_action_extended()
     with patch(