Cursors are not sent to the server correctly

[nbarbettini]

Describe the bug
The PaginatedRequest model has the cursor parameter in the wrong place: request.cursor instead of request.params.cursor as defined by the pagination spec.

Since the methods updated in #718 pass the cursor parameter down to the PaginatedRequest model, they are also affected by this bug.

To Reproduce

from mcp import types

list_tools_request = types.ListToolsRequest(
    method="tools/list",
    cursor="123",
)

client_request = types.ClientRequest(list_tools_request)

# Dump to JSON
client_request_json = client_request.model_dump_json()
print(client_request_json)

❯ python test.py
{"method":"tools/list","params":null,"cursor":"123"}

Expected behavior
A request that conforms to the pagination spec:

{"method":"tools/list","params":{"cursor":"123"}}