Fix pagination handling in McpServer

[stephentoub]

• We were defeating the purpose of pagination by doing all of the aggregation in the server. If a custom handler returns a paginated result, we should instead propagate that back to the client, who can choose to get more results when needed.
• We were adding tools/prompts from the collections on every request, even if there was a cursor. If multiple requests came in with different cursors, we'd re-add the same tools each time.