Unlist get_merge_request_notes

What does this MR do and why?

This MR marks the get_merge_request_notes MCP tool as unlisted.

An unlisted tool no longer shows up in the MCP tools/list response or in the AI Catalog tool picker. It still works when called directly through tools/call. So any existing caller keeps working.

Use get_merge_request with include: ["notes"] instead. This follows the same pattern as !254299 (merged), which unlisted get_workitem_notes.

We could not do this before, because the notes facet was missing some data. !258219 (merged) (merged) closed that gap. It added backward paging (notes_last, notes_before), each note's position and discussion, and the resolvedDiscussionsCount / resolvableDiscussionsCount counts. The facet now returns everything the old tool returned.

Changes in this MR:

• Add an unlisted? override on the service.
• Update docs: add a history entry, add a "superseded" note on the tool, and point the list_merge_requests notes paragraph to include: ["notes"].
• Update CE and EE list_tools request specs: drop the tool from the listed tools, and check it still can be called. One alias spec now uses the list_merge_requests alias, because the notes tool is no longer listed.
• Add the tool to the expected unlisted set in the AI Catalog mcp_tool_spec.
• Add a service spec check for unlisted?.

References

• Work item: #622712 (closed)
• Same pattern as: !254299 (merged)
• Blocker fixed by: !258219 (merged)

Screenshots or screen recordings

Not applicable. This change is backend and docs only. There is no UI.

How to set up and validate locally

Run these specs:

bundle exec rspec spec/requests/api/mcp/handlers/list_tools_spec.rb ee/spec/requests/api/mcp/handlers/list_tools_spec.rb ee/spec/models/ai/catalog/mcp_tool_spec.rb spec/services/mcp/tools/merge_requests/get_merge_request_notes_service_spec.rb

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