Serialize Time/Date in custom webhook template payloads

What does this MR do and why?

Custom webhook template rendering produced inconsistent JSON between the synchronous and asynchronous webhook paths.

WebHookService#render_custom_template calls value.to_json and strips the surrounding JSON quotes only when value.is_a?(String). Whether the renderer saw a Time/Date object or its String serialization depended on which path delivered the payload:

• The asynchronous path (#async_execute) enqueued the payload, Sidekiq JSON-roundtripped it, and the worker received Strings.
• The synchronous path (#execute, used by the Test webhook UI button) handed raw Time/Date objects straight to the renderer.

For a docs-compliant template such as "{{object_attributes.created_at}}" that meant the test path produced doubled quotes (""…"" — invalid JSON) while production worked.

For an unquoted template the symptom was the mirror image: production stripped the quotes (bare-timestamp — invalid JSON) while test mode worked by accident.

This MR adds a Gitlab::WebHooks.normalize_dates helper that recursively serializes Time, DateTime, ActiveSupport::TimeWithZone, and Date values to ISO 8601 strings (milliseconds for time, plain ISO date otherwise), and calls it in both places that feed the renderer:

• WebHookService#async_execute, before enqueuing to Sidekiq, so the worker receives a controlled string format instead of relying on Rails default to_json behavior.
• WebHookService#request_payload, before rendering a custom webhook template, so the synchronous test-mode path matches the asynchronous one. The call is idempotent on the async path (already stringified).

After this change, the docs-compliant pattern produces the same valid JSON in both modes:

{
  "created_at": "2026-04-23T12:30:45.123Z",
  "due_on": "2026-04-23"
}

References

• Resolves #584387 (closed)
• Supersedes !227597 (closed)

How to set up and validate locally

1. Create any project.

2. Go to the project's Settings → Webhooks and add a webhook (for example on issue events)

3. Create at least one issue in that project.

4. Use a docs-compliant custom payload template (placeholders wrapped in JSON double quotes):

   {
     "created_at": "{{object_attributes.created_at}}",
     "updated_at": "{{object_attributes.updated_at}}"
   }

5. Click Test → Issue events. The receiver should record a request with valid JSON, dates wrapped in quotes.

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