add a [JsonProperty("caption")] attribute to ensure proper serialization

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: KoditkarVedant

Src/Notion.Client/Models/File/FIleInput/ExternalFileInput.cs

@@ -11,6 +11,7 @@ public class ExternalFileInput : IFileObjectInput
         [JsonProperty("name")]
         public string Name { get; set; }
 
+        [JsonProperty("caption")]
         public IEnumerable<RichTextBaseInput> Caption { get; set; }
 
         public class Data