Add cursor pagination to chat sessions and messages

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What to build

Chat sessions and chat messages paginate using the estate cursor convention: an opaque cursor and a limit going in, an items array and a next cursor coming back.

These are the collections in this service that genuinely grow without bound. Bounded collections — workspaces, repositories, projects, credentials — are deliberately left alone, because turning every list into an envelope is a breaking change across the estate for collections that hold single digits.

The cursor shape is chosen over page and size because it stays stable under concurrent inserts, which is what an infinitely scrolling message list needs.

Acceptance criteria

• Both collections accept a cursor and a limit and return a next cursor
• Paging through a collection while items are being inserted neither duplicates nor skips
• The convention is written down somewhere a future service can copy
• The breaking change to these two operations is deliberate and their consumers are updated in step

Blocked by

• Adopt the customizer and annotate the domain error codes