Support --no-allow-remote-refs via BooleanOptionalAction

Use BooleanOptionalAction (like --use-annotated) so users can
explicitly opt out with --no-allow-remote-refs from the CLI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: butvinm

src/datamodel_code_generator/arguments.py

@@ -129,9 +129,10 @@ def start_section(self, heading: str | None) -> None:
     "--allow-remote-refs",
     help="Allow fetching remote $ref references over HTTP/HTTPS. "
     "Currently remote fetching is allowed by default but emits a deprecation warning. "
-    "Pass this flag explicitly to silence the warning. "
+    "Pass --allow-remote-refs to opt in without warning, "
+    "or --no-allow-remote-refs to block remote fetching. "
     "In a future version, remote fetching will be disabled by default.",
-    action="store_true",
+    action=BooleanOptionalAction,
     default=None,
 )
 base_options.add_argument(