[Schema Inaccuracy] code_scanning_alert closed_by_user webhook: fixed_at typed as null instead of date-time string

[donicrosby]

Expected

In the code_scanning_alert webhook event with action: "closed_by_user", the alert.fixed_at property should be typed as a nullable ISO 8601 date-time string:

fixed_at:
  type: string
  format: date-time
  nullable: true
  description: >-
    The time that the alert was fixed in ISO 8601 format: YYYY-MM-DDTHH:MM:SSZ.

This would be consistent with how fixed_at is already defined on:

• The REST API endpoint GET /repos/{owner}/{repo}/code-scanning/alerts/{alert_number}, where it is correctly typed as string or null with format: date-time.
• The code_scanning_alert webhook with action: "fixed" (corrected in [Schema Inaccuracy] code_scanning_alert fixed webhook: fixed_at typed as null instead of date-time string #6058).

Actual

The webhook schema for code_scanning_alert (action closed_by_user) defines fixed_at with only type: null, meaning it can never contain a value — only null or absent.

Reproduction Steps

1. Configure a repository webhook (or GitHub App) to receive code_scanning_alert events.
2. Have a code scanning alert that was previously auto-fixed (state: "fixed", fixed_at populated with a datetime).
3. A user closes (dismisses) the alert via the GitHub UI, triggering a code_scanning_alert webhook with action: "closed_by_user".
4. Inspect the webhook payload. The alert.fixed_at field contains an ISO 8601 datetime string, e.g. "2026-03-04T17:53:59Z".
5. Attempt to validate this payload against a client generated from the OpenAPI spec. Validation fails because the schema only permits null for fixed_at.

Impact

Any strongly-typed client generated from this spec (e.g., githubkit for Python, Octokit for TypeScript) will reject valid code_scanning_alert closed_by_user webhook payloads because fixed_at does not conform to the null-only schema.

Reference

• Previous fix for the same field on the fixed action: [Schema Inaccuracy] code_scanning_alert fixed webhook: fixed_at typed as null instead of date-time string #6058
• REST API endpoint schema (correct): https://docs.github.com/en/rest/code-scanning/code-scanning#get-a-code-scanning-alert
• Webhook event docs: https://docs.github.com/en/webhooks/webhook-events-and-payloads#code_scanning_alert