Fix pagination handling in McpServer (#177)

- We were adding tools/prompts from the collections on every request. If multiple requests came in with different cursors, we'd re-add the same tools each time.
- 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.

tests/ModelContextProtocol.Tests/Server/McpServerTests.cs

@@ -1,6 +1,5 @@
 using Microsoft.Extensions.AI;
 using Microsoft.Extensions.DependencyInjection;
-using ModelContextProtocol.Client;
 using ModelContextProtocol.Protocol.Messages;
 using ModelContextProtocol.Protocol.Transport;
 using ModelContextProtocol.Protocol.Types;