feat: add pagination and sorting to listSessions

[adwibha]

What this does

Adds pagination and sorting to the listSessions function, as requested in #324.

You can now:

• Sort sessions by last update time (ascending or descending)
• Paginate results with limit and offset, or use page for convenience
• Get metadata back (total count, total pages, current page)

And it's backward compatible, existing code that doesn't use these features works exactly as before.

How it works

• When you pass limit, you get paginated results with metadata
• When you pass offset alone, it slices from that position
• When you pass neither, you get all sessions (old behavior)
• Sorting is optional, if you don't specify order, there's no sorting overhead

Both the database and in-memory implementations work the same way.

Link to Issue or Description of Change

1. Link to an existing issue:

• Closes: Pagination Limit and sorting for sessionService.listSessions #324

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Test results: 1306 tests passed (23 new tests added for pagination and sorting)

Test Files  116 passed (116)
Tests  1306 passed (1306)

Manual testing:
Direct service calls work as expected:

// Paginated
await service.listSessions({ appName, userId, limit: 10, page: 1, order: 'desc' })
// Returns: { sessions: [...], page: 1, limit: 10, totalItems: N, totalPages: M }

// Backward compatible
await service.listSessions({ appName, userId })
// Returns: { sessions: [...], page: 1, limit: allCount, totalItems: N, totalPages: 1 }

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my feature works.
• New and existing unit tests pass locally with my changes.

Additional context

This PR implements the core service layer fully per spec. The HTTP API layer (dev/src/server routes and AdkApiClient) to expose these features is intentionally left as a follow-up to keep this focused.

[adwibha]

I see core/src/sessions/vertex_ai_session_service.ts was merged while this PR was under review. I will rebase and include fix in this PR shortly. Thanks for your approval.

[adwibha]

Hello @kalenkevich,

Rebased onto latest main to incorporate vertex_ai_session_service.ts (which was merged after this PR was opened). Updated the implementation in that service to align with the new ListSessionsResponse contract and to correctly handle limit, offset, page and order parameters, previously it was returning the full session list without pagination applied.

Note: userId in ListSessionsRequest is currently marked as required, but all three service implementations treat it as optional. Leaving this as a follow-up to keep the scope of this PR focused.