Update get_merge_request tool to be graphQL backed + add new capabilities

What does this MR do and why?

Consolidates the merge request read tools on the GitLab MCP server. get_merge_request becomes the single facet reader: its associated data (commits, notes, pipelines, discussions, diff (stats only)) is requested through the include parameter, one facet per call (a single-value string, so each call stays a single bounded query).

NOTEs:

• No standalone MCP tools are removed, that will be handled separately (see #605878 (comment 3647801471))
• The deprecated id alias is dropped in favor of project_id
• diff is stats only and conflicts is deferred due to some graphQL work required to support them both

References

• contributes to #605878 (closed)
• Epic gitlab-org/gitlab&22781

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. Restart the Rails web process so the tool registry picks up the changes:

   gdk restart rails-web

2. Connect using MCP inspector and list the MCP tools and confirm no tools are missing.

   npx -y @modelcontextprotocol/inspector -- env NODE_TLS_REJECT_UNAUTHORIZED=0 mise x -- npx -y mcp-remote https://gdk.test:3443/api/v4/mcp --debug

3. Call get_merge_request with a facet (one per call) and confirm the data comes back inline:

4. Call get_merge_request with no facet data comes back:

5. Call it with multiple facets, you get a validation error:

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist.