slack_incoming: Add ok=false to JSON in case of error.

Previously, errors were returned using Zulip's default format,
which did not match Slack's expected response structure.

This change ensures that errors in the Slack incoming webhook handler
return JSON responses in Slack's expected format: {ok: false, error:
"error string"}.

Fixes: #31878.

zerver/webhooks/slack_incoming/tests.py

@@ -66,7 +66,8 @@ Hello, world.
     def test_message_without_payload(self) -> None:
         self.url = self.build_webhook_url()
         result = self.client_post(self.url)
-        self.assert_json_error(result, "Missing 'payload' argument")
+        self.assertEqual(result.json()["error"], "Missing 'payload' argument")
+        self.assertEqual(result.json()["ok"], False)
 
     def test_message_with_actions(self) -> None:
         expected_topic_name = "C1H9RESGL"

zerver/webhooks/slack_incoming/view.py

@@ -1,10 +1,13 @@
 # Webhooks for external integrations.
 import re
+from collections.abc import Callable
+from functools import wraps
 from itertools import zip_longest
 from typing import Literal, TypedDict, cast
 
-from django.http import HttpRequest, HttpResponse
+from django.http import HttpRequest, HttpResponse, JsonResponse
 from django.utils.translation import gettext as _
+from typing_extensions import ParamSpec
 
 from zerver.decorator import webhook_view
 from zerver.lib.exceptions import JsonableError
@@ -25,9 +28,31 @@ from zerver.lib.validator import (
 from zerver.lib.webhooks.common import OptionalUserSpecifiedTopicStr, check_send_webhook_message
 from zerver.models import UserProfile
 
+ParamT = ParamSpec("ParamT")
+
+
+def slack_error_handler(view_func: Callable[..., HttpResponse]) -> Callable[..., HttpResponse]:
+    """
+    A decorator that catches JsonableError exceptions and returns a
+    Slack-compatible error response in the format:
+    {ok: false, error: "error message"}.
+    """
+
+    @wraps(view_func)
+    def wrapped_view(
+        request: HttpRequest, *args: ParamT.args, **kwargs: ParamT.kwargs
+    ) -> HttpResponse:
+        try:
+            return view_func(request, *args, **kwargs)
+        except JsonableError as error:
+            return JsonResponse({"ok": False, "error": error.msg}, status=error.http_status_code)
+
+    return wrapped_view
+
 
 @webhook_view("SlackIncoming")
 @typed_endpoint
+@slack_error_handler
 def api_slack_incoming_webhook(
     request: HttpRequest,
     user_profile: UserProfile,