Add cursor-based pagination for messages

[musonza]

Summary

• Adds cursor-based pagination as an alternative to offset-based pagination for message retrieval
• Cursor pagination prevents duplicate messages when new messages arrive between page loads
• Maintains backwards compatibility by adding a new endpoint rather than modifying the existing one

New Endpoint

GET /conversations/{id}/messages-cursor

Parameters:

• participant_id (required)
• participant_type (required)
• perPage (optional, default: 25)
• sorting (optional: asc/desc)
• cursor (optional - from previous response)

Programmatic Usage

Chat::conversation($conversation)
    ->setParticipant($user)
    ->setCursorPaginationParams(['perPage' => 25, 'cursor' => $nextCursor])
    ->getMessagesWithCursor();

Test plan

• Basic cursor pagination returns correct response structure
• Navigation through multiple pages using cursor works correctly
• Descending sort order works as expected
• Required parameter validation works

Closes #324