Add keyset pagination for project/issues endpoint

Add Keyset Pagination to Project Issues API Endpoint

What does this MR do and why?

This MR adds keyset-based pagination support to the project issues API endpoint (GET /projects/:id/issues). Keyset pagination provides more efficient and stable pagination compared to offset-based pagination, especially for large datasets.

Key Changes

• Adds keyset pagination support for the project issues endpoint
• Supports ordering by: id, title, created_at, updated_at, due_date, relative_position, and weight
• Excludes complex order options that require table joins: label_priority, popularity, milestone_due, and priority
• Falls back to offset pagination for unsupported ordering options

Implementation Details

1. Model Changes (app/models/issue.rb):

   • Added supported_keyset_orderings class method defining which fields support keyset pagination

2. API Changes (lib/api/issues.rb):

   • Added cursor parameter support for keyset pagination
   • Uses paginate_with_strategies to handle both keyset and offset pagination
   • Automatically falls back to offset pagination for unsupported order options

3. Documentation Updates (doc/api/issues.md, doc/api/rest/_index.md):

   • Added cursor parameter documentation
   • Updated REST API index to include project issues in the list of keyset-paginated endpoints

References

gitlab-com/gl-infra/production-engineering#26555 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Make a request to the project issues endpoint with keyset pagination:

   curl -H "PRIVATE-TOKEN: your-token" \
    "http://localhost:3000/api/v4/projects/1/issues?pagination=keyset&per_page=2&order_by=created_at"

2. Check the response headers for the Link header containing the cursor:

   Link: <http://localhost:3000/api/v4/projects/1/issues?pagination=keyset&per_page=2&order_by=created_at&cursor=eyJjcmVhdGVkX2F0IjoiMjAyNC0wMS0wMSIsImlkIjoxMjM0fQ%3D%3D>; rel="next"

3. Use the cursor from the Link header in the next request:

   curl -H "PRIVATE-TOKEN: your-token" \
    "http://localhost:3000/api/v4/projects/1/issues?pagination=keyset&per_page=2&order_by=created_at&cursor=eyJjcmVhdGVkX2F0IjoiMjAyNC0wMS0wMSIsImlkIjoxMjM0fQ%3D%3D"

4. Verify that:

   • Different pages of results are returned
   • The cursor changes between requests
   • No cursor is returned on the last page
   • Unsupported order options (like label_priority) fall back to offset pagination

MR acceptance checklist

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