Simplify pagination

[ronaldmannak]

This PR simplifies pagination by adding a straightforward way to fetch subsequent pages.

I initially suspected I’d found a bug (see #17), but it turned out to be confusion caused by the lack of an end-to-end example and the way nextURL behaves. This PR clarifies the intended flow and makes “get the next page” a first-class operation.

Example usage:

var page = try await client.listModels(limit: 100)
print("Page 1: \(page.items.count) models")

while page.nextURL != nil {
    guard let next = try await client.nextPage(of: page) else { break }
    page = next
    print("Page: \(page.items.count) models")
}