[IJAI-1134] feat(agent,protocol): support suspend-based paginated session/list

[forketyfork]

Issue

https://youtrack.jetbrains.com/issue/IJAI-1134

session/list (and any other AcpPaginatedRequest/AcpPaginatedResponse method) is served through setPaginatedRequestHandler, backed by SequenceToPaginatedResponseAdapter. That adapter needs a non-suspending Sequence, so an agent whose session source is a database can only satisfy the contract by materializing the entire result set inside the one suspend call it's given, before pagination even starts. The adapter also keeps the resulting iterator in server-process memory behind a cursor that is single-use and evicted after a minute of inactivity — a real cost for a client that scrolls slowly or reconnects, since its cursor is simply gone.

In our cloud-based implementation, we need a session/list that walks a keyset straight from the DB, one page per request, with a cursor that stays valid indefinitely and survives a reconnect.

Solution

• setSuspendPaginatedRequestHandler (Protocol.extensions.kt): a sibling of setPaginatedRequestHandler that takes pageFactory: suspend (request) -> Pair<List<TItem>, String?> instead of a Sequence. No iterator or cursor state is kept by the SDK — the cursor is opaque and entirely owned by whatever pageFactory encodes into it, so it can be as durable as a database keyset, and a client that pauses or reconnects loses nothing.
• AgentSupport.listSessionsPage (AgentSupport.kt): a new optional hook alongside listSessions, defaulting to null ("not overridden"). Agent now tries this first for session/list; when it returns non-null, the response is served directly from it via the new suspend-based path. When it returns null (the default), session/list falls back to listSessions()/Sequence exactly as before — existing agents that only override listSessions() see no behavior change.

Context

• Happy to adjust the API or shape of listSessionsPage (e.g. request-object vs. individual params, Pair vs. a small Page type) based on review.
• No existing test exercised session/list end-to-end through Agent before this PR (only SequenceToPaginatedResponseAdapter was unit-tested directly); AgentSessionListTest adds that coverage for both the new and the fallback path.