Add list_projects MCP tool

Radek Antoniukrequested to merge
gitlab-community/gitlab-org/gitlab:rantoniuk/591304-add-mcp-list-projects into master

What does this MR do and why?

The GitLab MCP server has no primitive for listing projects. Agents building a project picker, or trying to resolve which project a user means, have to either hard code IDs and paths or fall back to the generic search tool, which requires a search term and cannot filter by group, visibility, or archived state.

This adds a GraphQL-backed list_projects tool, built on Query.projects (Resolvers::ProjectsResolver), supporting optional group scoping (group_id, accepting either a numeric ID or a full path), search, visibility, and archived filters, with cursor pagination (first/after).

References

#591304 (closed)

How to set up and validate locally

  1. Restart rails-web so Mcp::Tools::Manager picks up the new registration.
  2. Call tools/list on POST /api/v4/mcp with a personal access token that has the mcp scope, and confirm list_projects is advertised.
  3. Call tools/call with { "name": "list_projects", "arguments": { "group_id": "gitlab-org", "first": 5 } } and confirm it returns projects under that group.

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