Add get_merge_request_notes MCP tool

What does this MR do and why?

Adds a get_merge_request_notes MCP tool that returns a merge request's notes (comments and system notes) to MCP clients. It accepts either a url or a project_id + merge_request_iid, and paginates over the flat notes connection the same way get_work_item_notes does. Each note carries its discussion (id plus resolution state) so an agent can group threads and spot unresolved ones, and the response also includes resolvedDiscussionsCount / resolvableDiscussionsCount for an at-a-glance summary.

Design notes — query shape, naming, follow-ups

Output shape: the tool queries the flat notes connection on the merge request (mirroring get_work_item_notes) and returns the merge request with the two resolution counts plus the paginated notes connection. Pagination is over notes, so each note includes its discussion { id } for grouping. Filtering system notes server-side needs a change to the merge request discussions GraphQL field and is deferred (see follow-up below).

Query location: the GraphQL document lives in app/graphql/queries/mcp/merge_requests/get_merge_request_notes.query.graphql rather than an inline heredoc, so spec/graphql/all_queries_spec.rb validates it against GitlabSchema in CI. The load_graphql helper added to GraphqlTool makes this reusable for future tools.

Service naming: the new service is GetMergeRequestNotesService — deliberately without the Graphql prefix the existing GRAPHQL_TOOLS services carry. The prefix carries no signal: tools group by domain, the paired Tool class is unprefixed, and the class already inherits from GraphqlService.

The CE and EE tools/list request specs are updated to include the new tool.

References

• Related to #597494 (closed) (the issue also covers create_merge_request_note; this MR delivers the read tool only)
• Follow-up: #603390 (add a filter argument to the merge request discussions GraphQL field, enabling system-note filtering and thread-based pagination)
• Follow-up: #603389 (closed) (move remaining MCP tool queries into .graphql files)
• Follow-up: #603096 (closed) (reorganize Mcp::Tools by domain, drop Graphql prefix)
• Follow-up: #603097 (generate the MCP tools reference doc from tool metadata)

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. In a Rails console (or bin/rails runner), fetch the tool and set credentials:

   tool = Mcp::Tools::Manager.new.get_tool(name: 'get_merge_request_notes')
   tool.set_cred(current_user: User.find_by_username('root'), access_token: nil)

2. Fetch by project and IID:

   tool.execute(params: { arguments: { project_id: 'group/project', merge_request_iid: 1 } })

3. Fetch by URL instead (works for group/subgroup paths):

   tool.execute(params: { arguments: { url: 'https://gdk.test:3443/group/project/-/merge_requests/1' } })

4. Confirm isError is false and structuredContent contains resolvedDiscussionsCount, resolvableDiscussionsCount, and a notes connection (pageInfo + count + nodes), with each node carrying a discussion.
5. Check the error messages: a bad project_id returns Project not found: …; a valid project with a missing merge_request_iid returns Merge request not found: ….
6. Run the specs:

   bundle exec rspec spec/services/mcp/tools/merge_requests/ spec/requests/api/mcp/handlers/list_tools_spec.rb ee/spec/requests/api/mcp/handlers/list_tools_spec.rb
   bundle exec rspec spec/graphql/all_queries_spec.rb -e 'mcp/merge_requests/get_merge_request_notes'

using @modelcontextprotocol/inspector

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

example output (real output from a GDK merge request, truncated to two notes)

{
  "resolvedDiscussionsCount": 0,
  "resolvableDiscussionsCount": 3,
  "notes": {
    "pageInfo": {
      "hasNextPage": false,
      "hasPreviousPage": false,
      "startCursor": "eyJjcmVhdGVkX2F0IjoiMjAyNi0wNi0xMCAxNzoyNjo1NS4wODkwNDUwMDAgKzAwMDAiLCJpZCI6IjE1NDQifQ",
      "endCursor": "eyJjcmVhdGVkX2F0IjoiMjAyNi0wNi0xNiAxMzoxMTozNi40MzE1MTMwMDAgKzAwMDAiLCJpZCI6IjE1NDgifQ"
    },
    "count": 5,
    "nodes": [
      {
        "id": "gid://gitlab/DiscussionNote/1544",
        "webUrl": "https://gdk.test:3443/top-level-public/sub-group-internal/sub-group-private/sub-project-private/-/merge_requests/1#note_1544",
        "body": "test 1",
        "system": false,
        "internal": false,
        "createdAt": "2026-06-10T17:26:55Z",
        "updatedAt": "2026-06-10T17:26:59Z",
        "author": {
          "id": "gid://gitlab/User/1",
          "username": "root",
          "name": "Administrator"
        },
        "position": null,
        "discussion": {
          "id": "gid://gitlab/Discussion/16332cf01fa2c27f4b5d1abf29ffa2f740da78fc",
          "resolvable": true,
          "resolved": false,
          "resolvedBy": null
        }
      },
      {
        "id": "gid://gitlab/DiffNote/1548",
        "webUrl": "https://gdk.test:3443/top-level-public/sub-group-internal/sub-group-private/sub-project-private/-/merge_requests/1#note_1548",
        "body": "```suggestion:-1+0\n# Test\n\nAdd more data here\n\nThis is not enough\n```",
        "system": false,
        "internal": false,
        "createdAt": "2026-06-16T13:11:36Z",
        "updatedAt": "2026-06-16T13:11:36Z",
        "author": {
          "id": "gid://gitlab/User/1",
          "username": "root",
          "name": "Administrator"
        },
        "position": {
          "newPath": "test.md",
          "oldPath": "test.md",
          "newLine": 2,
          "oldLine": null
        },
        "discussion": {
          "id": "gid://gitlab/Discussion/6ec8153959f587b9713de1f3c5de6987a0e852e4",
          "resolvable": true,
          "resolved": false,
          "resolvedBy": null
        }
      }
    ]
  }
}

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