fix(golden): make --check and --diff mutually exclusive

The parser only rejected --check + --new and --diff + --new. Passing
both --check and --diff was accepted, with --diff winning on output.
The docstring describes them as alternatives, so reject the combination
at the parser — symmetric with the existing --new checks, and less
surprise.

Author: bnusunny

tests/golden/test_update_goldens.py

@@ -102,3 +102,14 @@ def boom(*_args, **_kwargs):
     monkeypatch.setattr(update_goldens, "_generate", boom)
     rc = update_goldens.main(["--new"])
     assert rc == 0
+
+
+def test_check_and_diff_are_mutually_exclusive(capsys):
+    """--check and --diff are documented as alternatives. Reject the
+    combination at the parser, symmetric with the existing --new mutex
+    checks."""
+    with pytest.raises(SystemExit) as excinfo:
+        update_goldens.main(["--check", "--diff"])
+    assert excinfo.value.code == 2  # argparse parser.error exits 2
+    err = capsys.readouterr().err
+    assert "--check and --diff are mutually exclusive" in err

tests/golden/update_goldens.py

@@ -106,6 +106,12 @@ def main(argv: Optional[List[str]] = None) -> int:
         parser.error("--check and --new are mutually exclusive")
     if args.diff and args.new:
         parser.error("--diff and --new are mutually exclusive")
+    if args.check and args.diff:
+        # The implementation tolerates this (both imply dry_run, --diff
+        # wins on output), but the docstring describes them as
+        # alternatives. Reject at the parser, symmetric with the --new
+        # checks above.
+        parser.error("--check and --diff are mutually exclusive")
 
     dry_run = args.check or args.diff
     any_diff = False