[FEATURE]: add cursor/offset pagination to GET /session/:id/message #20111

Feature hasn't been suggested before.

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Describe the enhancement you want to request

GET /session/:id/message only supports a limit query parameter. There is no way to page backwards through history — you can fetch the last N messages, but you cannot fetch messages before a given message ID or offset.

For clients that want to show a paginated conversation view (e.g. load the last 10 messages on open, then let the user load earlier history), this means the only option is to fetch everything or accept a fixed truncation.

Long sessions with heavy tool use can produce very large payloads — easily hundreds of MB for a single session fetch. Without pagination, clients are forced to either transfer all of it or show incomplete history with no way to load more in a controlled way.

Proposed solution:

Add a before cursor parameter (message ID) to the endpoint:

GET /session/:id/message?limit=20&before=<messageID>

This would return the 20 messages immediately preceding <messageID>, enabling incremental backwards pagination.

A before cursor is preferable over a simple integer offset since message counts can change (e.g. after compaction), but either would be a meaningful improvement over the current state.