Add approvals facet to the get_merge_request MCP tool

What does this MR do and why?

Adds approvals to the include enum of the existing get_merge_request MCP server tool. It does not add a new tool. This replaces !250827 (closed), which proposed a standalone get_merge_request_approvals tool.

How to test locally

1. Call the tool in rails runner on a GDK with an Ultimate license, against an open merge request you can read. A call with no include returns no approval fields. A call with include: ['approvals'] returns the five fields.

   user = User.find_by(username: 'root')
   mr = MergeRequest.where(state_id: 1).last
   Mcp::Tools::MergeRequests::GetMergeRequestTool.new(
     current_user: user,
     params: { project_id: mr.target_project.id.to_s, merge_request_iid: mr.iid, include: ['approvals'] }
   ).execute

2. Verified output on an Ultimate GDK, for a merge request with one approval rule that needs 2 approvals and has 1 approval recorded:

   approved=true  approvalsRequired=2  approvalsLeft=1
   approvedBy.nodes=[{id, username, name} for the one approver]
   approvalState.rules=[{name, type: REGULAR, approvalsRequired: 2, approved: true, eligibleApprovers, approvedBy}]

Without a Premium or Ultimate license, the same call returns approved and approvedBy, and the other three fields come back empty.

Closes #596590 (closed)