fix(tools): handle non-dict JSON values in _parse_tool_args (closes #5043) (#5064)

When an LLM generates a valid JSON string that parses to a native non-dict
type (like a list, int, or string), _parse_tool_args previously returned
that object. Callers expecting a dictionary would then crash with
AttributeError or KeyError when attempting to look up action keys.

- Update _parse_tool_args in src/tool_utils.py to explicitly type-check
  the parsed JSON object and return {} for non-dict objects.
- Add test coverage in tests/test_admin_tools_registry.py for lists,
  ints, and strings.

tests/test_admin_tools_registry.py

@@ -72,3 +72,8 @@ def test_parse_tool_args_lives_in_tool_utils_single_source():
     assert _parse_tool_args('{"action":"add"}') == {"action": "add"}
     # body-envelope unwrap still works
     assert _parse_tool_args('{"body":{"action":"x"}}') == {"action": "x"}
+
+    # non-dict JSON values should return {}
+    assert _parse_tool_args('[1, 2]') == {}
+    assert _parse_tool_args('42') == {}
+    assert _parse_tool_args('"hello"') == {}