feat: implement ListAllModelsAsync and ListModelsAsync

src/AnthropicClient/Models/Page.cs

@@ -11,13 +11,13 @@ public class Page
   /// The id of the first item in the page.
   /// </summary>
   [JsonPropertyName("first_id")]
-  public string FirstId { get; init; } = string.Empty;
+  public string? FirstId { get; init; } = string.Empty;
 
   /// <summary>
   /// The id of the last item in the page.
   /// </summary>
   [JsonPropertyName("last_id")]
-  public string LastId { get; init; } = string.Empty;
+  public string? LastId { get; init; } = string.Empty;
 
   /// <summary>
   /// Indicates whether there is more data to be retrieved.

src/AnthropicClient/Models/PagingRequest.cs

@@ -70,10 +70,7 @@ public class PagingRequest
       parameters.Add($"after_id={AfterId}");
     }
 
-    if (Limit is not DefaultLimit)
-    {
-      parameters.Add($"limit={Limit}");
-    }
+    parameters.Add($"limit={Limit}");
 
     return string.Join("&", parameters);
   }