Add checks for message content intent on bot accounts

Related to #918

DiscordChatExporter.Core/Discord/Data/Message.cs

@@ -10,7 +10,7 @@ using JsonExtensions.Reading;
 namespace DiscordChatExporter.Core.Discord.Data;
 
 // https://discord.com/developers/docs/resources/channel#message-object
-public record Message(
+public partial record Message(
     Snowflake Id,
     MessageKind Kind,
     User Author,
@@ -26,6 +26,16 @@ public record Message(
     IReadOnlyList<User> MentionedUsers,
     MessageReference? Reference,
     Message? ReferencedMessage) : IHasId
+{
+    public bool IsEmpty =>
+        Kind == MessageKind.Default &&
+        string.IsNullOrEmpty(Content) &&
+        !Attachments.Any() &&
+        !Embeds.Any() &&
+        !Stickers.Any();
+}
+
+public partial record Message
 {
     private static IReadOnlyList<Embed> NormalizeEmbeds(IReadOnlyList<Embed> embeds)
     {

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -323,6 +323,11 @@ public class DiscordClient
                 if (message.Timestamp > lastMessage.Timestamp)
                     yield break;
 
+                // Make sure the messages are not empty when exporting via a bot
+                // https://github.com/Tyrrrz/DiscordChatExporter/issues/918
+                if (_resolvedTokenKind == TokenKind.Bot && message.IsEmpty && !message.Author.IsBot)
+                    throw DiscordChatExporterException.MessageContentIntentMissing();
+
                 // Report progress based on the duration of exported messages divided by total
                 if (progress is not null)
                 {

DiscordChatExporter.Core/Exceptions/DiscordChatExporterException.cs

@@ -41,4 +41,7 @@ Failed to perform an HTTP request.
 
     internal static DiscordChatExporterException ChannelIsEmpty() =>
         new("No messages found for the specified period.");
+
+    internal static DiscordChatExporterException MessageContentIntentMissing() =>
+        new("Failed to retrieve message content because the bot doesn't have the Message Content Intent enabled.");
 }